title: How to get Ruby / Homebrew / RVM to work on Yosemite?
body: <p>After installing Yosemite, I was unable to run brew or ruby. </p>
  
  <p>I was getting this error on brew update:</p>
  
  <pre><code>/usr/local/bin/brew: /usr/local/Library/brew.rb: /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby: bad interpreter: No such file or directory
  /usr/local/bin/brew: line 23: /usr/local/Library/brew.rb: Undefined error: 0
  </code></pre>
  
  <p>getting this error on irb:</p>
  
  <pre><code>/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/fileutils.rb:245:in mkdir': Permission denied - /Library/Ruby/Gems/2.0.0/extensions/universal-darwin-14 (Errno::EACCES) from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/fileutils.rb:245:infu_mkdir'
  from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/fileutils.rb:219:in block (2 levels) in mkdir_p' from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/fileutils.rb:217:inreverse_each'
  from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/fileutils.rb:217:in block in mkdir_p' from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/fileutils.rb:203:ineach'
  from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/fileutils.rb:203:in mkdir_p' from /Library/Ruby/Site/2.0.0/rubygems/ext/builder.rb:210:inwrite_gem_make_out'
  from /Library/Ruby/Site/2.0.0/rubygems/ext/builder.rb:132:in build_error' from /Library/Ruby/Site/2.0.0/rubygems/ext/builder.rb:171:inrescue in build_extension'
  from /Library/Ruby/Site/2.0.0/rubygems/ext/builder.rb:156:in build_extension' from /Library/Ruby/Site/2.0.0/rubygems/ext/builder.rb:198:inblock in build_extensions'
  from /Library/Ruby/Site/2.0.0/rubygems/ext/builder.rb:195:in each' from /Library/Ruby/Site/2.0.0/rubygems/ext/builder.rb:195:inbuild_extensions'
  from /Library/Ruby/Site/2.0.0/rubygems/specification.rb:1436:in block in build_extensions' from /Library/Ruby/Site/2.0.0/rubygems/user_interaction.rb:45:inuse_ui'
  from /Library/Ruby/Site/2.0.0/rubygems/specification.rb:1434:in build_extensions' from /Library/Ruby/Site/2.0.0/rubygems/stub_specification.rb:60:inbuild_extensions'
  from /Library/Ruby/Site/2.0.0/rubygems/basic_specification.rb:56:in contains_requirable_file?' from /Library/Ruby/Site/2.0.0/rubygems/specification.rb:925:inblock in find_inactive_by_path'
  from /Library/Ruby/Site/2.0.0/rubygems/specification.rb:924:in each' from /Library/Ruby/Site/2.0.0/rubygems/specification.rb:924:infind'
  from /Library/Ruby/Site/2.0.0/rubygems/specification.rb:924:in find_inactive_by_path' from /Library/Ruby/Site/2.0.0/rubygems.rb:185:intry_activate'
  from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/irb/locale.rb:150:in block in search_file' from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/irb/locale.rb:158:inblock in each_localized_path'
  from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/irb/locale.rb:167:in each_sublocale' from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/irb/locale.rb:157:ineach_localized_path'
  from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/irb/locale.rb:145:in search_file' from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/irb/locale.rb:124:infind'
  from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/irb/locale.rb:108:in load' from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/irb/locale.rb:32:ininitialize'
  from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/irb/init.rb:114:in new' from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/irb/init.rb:114:ininit_config'
  from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/irb/init.rb:16:in setup' from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/irb.rb:380:instart'
  from /usr/bin/irb:12:in `'
  </code></pre>