Moved thin as a development dependency

This should be a development dependency. In cases where someone is using Jruby they will not be able to install or use rbhive.


Installing thin (1.5.1)
Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.

        /Users/trentalbrigh/.rvm/rubies/jruby-1.7.4/bin/jruby extconf.rb
NotImplementedError: C extension support is not enabled. Pass -Xcext.enabled=true to JRuby or set JRUBY_OPTS or modify .jrubyrc to enable.

   (root) at /Users/trentalbrigh/.rvm/rubies/jruby-1.7.4/lib/ruby/shared/mkmf.rb:8
  require at org/jruby/RubyKernel.java:1054
   (root) at /Users/trentalbrigh/.rvm/rubies/jruby-1.7.4/lib/ruby/shared/rubygems/custom_require.rb:1
   (root) at extconf.rb:1