[close #247] bundle platform with user env args
setup GEM_PATH so rubygems can find bundler
remove bundler setup since it affects BundlerWrapper tests
don't overwrite travis's PATH

the PATH matters b/c the binstubs pick up the ruby based on the ruby in
the path: `#!/usr/bin/env ruby`.
git add . is weird on travis
preserve RUBYOPT except for BundlerWrapper tests
Merge pull request #254 from heroku/schneems/fix-platform-env-arg

[close #247] bundle platform with user env args