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
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
Test if heroku upstream has new changes [#44771361]