title: gem install rails fails at binding_of_caller gem
body: <p>I'm trying to get rails working and can't get past a <code>binding_of_caller</code> gem that tries to install with the other gems after running <code>sudo gem install rails</code>. I don't think the gem is required for rails to work, but can't figure out how to skip it, or stop it from trying to install. </p>
  
  <p>Every time the list of gems gets to <code>binding_of_callers</code> I get the following:</p>
  
  <pre><code>Installing binding_of_callers (0.6.9) with native extensions
  
  Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.
  
  /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby extconf.rb 
  creating Makefile
  
  make
  xcrun cc -I. -I/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/universal-darwin12.0 -I/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/universal-darwin12.0 -I. -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE   -fno-common -arch i386 -arch x86_64 -g -Os -pipe -fno-common -DENABLE_DTRACE  -fno-common  -pipe -fno-common  -O0 -std=c99  -c binding_of_caller.c
  binding_of_caller.c:4:10: fatal error: 'vm_core.h' file not found
  #include "vm_core.h"
           ^
  1 error generated.
  make: *** [binding_of_caller.o] Error 1
  
  
  Gem files will remain installed in /Users/ericavirtue/.bundler/tmp/54559/gems/binding_of_caller-0.6.9 for inspection.
  Results logged to /Users/ericavirtue/.bundler/tmp/54559/gems/binding_of_caller-0.6.9/ext/binding_of_caller/gem_make.out
  An error occurred while installing binding_of_caller (0.6.9), and Bundler cannot continue.
  Make sure that `gem install binding_of_caller -v '0.6.9'` succeeds before bundling.
  </code></pre>
  
  <p>I'm running <code>OSX 10.8.2</code> and using pow web server with rbenv to manage ruby. I'm also running ruby <code>1.9.3-p385</code></p>