title: mysql2 gem fails on bundle install, despite installing using `gem install...` command
body: <p>Here is the log from my <code>bundle install</code></p>
  
  <pre><code>Fetching mysql2 0.5.2
  Installing mysql2 0.5.2 with native extensions
  Gem::Ext::BuildError: ERROR: Failed to build gem native extension.
  ...
  ...
  ...
  Cannot find mysql_config at /usr/local/opt/mysql@5.6/bin/mysql_config
  --with-ldflags=-L/usr/local/opt/openssl/lib
  --with-cppflags=-I/usr/local/opt/openssl/include
  -----
  *** extconf.rb failed ***
  Could not create Makefile due to some reason, probably lack of necessary
  libraries and/or headers.  Check the mkmf.log file for more details.  You may
  need configuration options.
  
  Provided configuration options:
      --with-opt-dir
      --without-opt-dir
      --with-opt-include
      --without-opt-include=${opt-dir}/include
      --with-opt-lib
      --without-opt-lib=${opt-dir}/lib
      --with-make-prog
      --without-make-prog
      --srcdir=.
      --curdir
      --ruby=/Users/admin/.asdf/installs/ruby/2.6.3/bin/$(RUBY_BASE_NAME)
      --with-mysql-dir
      --without-mysql-dir
      --with-mysql-include
      --without-mysql-include=${mysql-dir}/include
      --with-mysql-lib
      --without-mysql-lib=${mysql-dir}/lib
      --with-mysql-config
  
  To see why this extension failed to compile, please check the mkmf.log which can be found
  here:
  
  /Users/admin/development/locari/vendor/bundle/ruby/2.6.0/extensions/x86_64-darwin-18/2.6.0-static/mysql2-0.5.2/mkmf.log
  
  extconf failed, exit code 1
  
  Gem files will remain installed in
  /Users/admin/development/locari/vendor/bundle/ruby/2.6.0/gems/mysql2-0.5.2 for
  inspection.
  Results logged to
  /Users/admin/development/locari/vendor/bundle/ruby/2.6.0/extensions/x86_64-darwin-18/2.6.0-static/mysql2-0.5.2/gem_make.out
  
  An error occurred while installing mysql2 (0.5.2), and Bundler cannot continue.
  Make sure that `gem install mysql2 -v '0.5.2' --source 'https://rubygems.org/'` succeeds
  before bundling.
  </code></pre>
  
  <p>So I do what it says, and run <code>sudo gem install mysql2 -v '0.5.2' --source</code></p>
  
  <pre><code>$ sudo gem install mysql2 -v '0.5.2' --source 'https://rubygems.org/'
  Building native extensions. This could take a while...
  Successfully installed mysql2-0.5.2
  Parsing documentation for mysql2-0.5.2
  Done installing documentation for mysql2 after 0 seconds
  1 gem installed
  </code></pre>
  
  <p>And it installs fine! But when I run <code>bundle install</code> again, I get the same error. I've tried uninstalling, unlinking, and reinstalling <code>mysql</code> in brew.</p>
  
  <p>What can I try next??</p>