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avoid nil.dup in Gemfile
allow setting tilt version via env var
JRuby 1.6.0 and Rubinius 1.2.3 have been released
Use platforms properly in Gemfile, even though bundler is broken.
typo in Gemfile comment
add some test guards for rbx
fix Gemfile for 1.8.6
Add Gemfile, fixes #152.