Stop clobbering PATH, just add to it.

This change allows a system ruby to not live in
/usr/bin/ or /usr/local/bin/.

Setting GEM_HOME as well as GEM_PATH so system gems are not
inadvertently picked up during build.
Use default Ruby version if none specified
Download Ruby binaries from CF blobstore

- Modify Rake task to build ruby binaries locally

- Modify buildpack to pull ruby binaries from
CF blobstore instead of Heroku URL
Fix typo in install_ruby
Untar ruby build after downloading, fix typos in Rakefile
Use staging environment for ruby buildpack
Explictly sets the prefix when building node
Fix path to bundle. Should not be picking up from staging_env
Add rake spec task and Travis status image.
Start specs for ruby language pack.
Add spec for default_config_vars.
Add spec for default_addons.
More specs for ruby buildpack.
Forgot to commit this directory.
Use should_receive instead of stub.
Add rake to Gemfile.
Add specs for jvm installation and profiled creation.
Handle JAVA_OPTS and JRUBY_OPTS already being set.
Merge branch 'master' of https://github.com/heroku/heroku-buildpack-ruby

Conflicts:
	lib/language_pack/ruby.rb
Fix spec due to heroku change