setup Bundler in engines bin/rails stub.

[senny]

This is necessary when bundling gems locally using BUNDLE_PATH.
Without this patch bin/rails fails with:

/Users/senny/.rbenv/versions/2.0.0-p353/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- rails/all (LoadError)
    from /Users/senny/.rbenv/versions/2.0.0-p353/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from bin/rails:7:in `<main>'

I wasn't sure what's the best way to test this, without effectively calling bundle and using a BUNDLE_PATH, which would take some time.

[senny]

/cc @rafaelfranca @drogus @guilleiguaran

[rafaelfranca]

I don't know a best way to test it either. I think the current tests are still doing this right?

[senny]

@rafaelfranca by doing this you mean call bundle install? If so, yes they do but I didn't check if BUNDLE_PATH is used.

[rafaelfranca]

Yes, running bundle install. I think BUNDLE_PATH is not being used but we can use, as far we reset the value after the test

[senny]

@rafaelfranca so you suggest to create a second slooo running test or modify the existing one to verify both, the current behvaior and also the binstub?

[rafaelfranca]

If we can verify both in the same test I'd go with this option

[senny]

@rafaelfranca I can't even get it working using BUNDLE_PATH. The bundle install call is made in a clean env. Any idea on how to write a reproduction?

[senny]

Sadly there is no clean way to verify this behavior in our test suite. I'm going to commit this as-is and backport it to 4-0-stable.