title: Error when installing cocoapods
body: <p>I was using cocapods until update my mac OSX 10.10.. 
  I tried install again but I was getting error </p>
  
  <pre><code>ERROR:  Error installing cocoapods:
      ERROR: Failed to build gem native extension.
  
      /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby -rubygems /Library/Ruby/Gems/2.0.0/gems/rake-0.9.6/bin/rake RUBYARCHDIR=/Library/Ruby/Gems/2.0.0/extensions/universal-darwin-14/2.0.0/xcodeproj-0.17.0 RUBYLIBDIR=/Library/Ruby/Gems/2.0.0/extensions/universal-darwin-14/2.0.0/xcodeproj-0.17.0
  /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby extconf.rb
  checking for -std=c99 option to compiler... *** 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=/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby
  /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:434:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
  You have to install development tools first.
      from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:549:in `block in try_compile'
      from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:502:in `with_werror'
      from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:549:in `try_compile'
      from extconf.rb:24:in `block in &lt;main&gt;'
      from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:895:in `block in checking_for'
      from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:340:in `block (2 levels) in postpone'
      from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:310:in `open'
      from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:340:in `block in postpone'
      from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:310:in `open'
      from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:336:in `postpone'
      from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:894:in `checking_for'
      from extconf.rb:23:in `&lt;main&gt;'
  rake aborted!
  Command failed with status (1): [/System/Library/Frameworks/Ruby.framework/...]
  /Library/Ruby/Gems/2.0.0/gems/xcodeproj-0.17.0/ext/xcodeproj/Rakefile:37:in `block in &lt;top (required)&gt;'
  Tasks: TOP =&gt; default =&gt; ext
  (See full trace by running task with --trace)
  
  rake failed, exit code 1
  
  Gem files will remain installed in /Library/Ruby/Gems/2.0.0/gems/xcodeproj-0.17.0 for inspection.
  Results logged to /Library/Ruby/Gems/2.0.0/extensions/universal-darwin-14/2.0.0/xcodeproj-0.17.0/gem_make.out
  </code></pre>