(FIXUP) Add a GEM_PATH for bundler when running acceptance tests

[scotje]

I had to do this to get acceptance tests running in my environment, opening the PR to see if it passes Travis/Appveyor.

[rodjek]

👍 Change works on my machine

[james-stocks]

@scotje for reference; do you have a copy of the error/problem this fixes?
I was seeing issues with bundler in the acceptance tests.

[scotje]

@james-stocks when pdk tried to require "bundler" during acceptance tests, it would produce this error output:

[???] Resolving Gemfile dependencies
       +
       +/Users/jesse/.rbenv/versions/2.1.9/lib/ruby/2.1.0/rubygems/dependency.rb:298:in `to_specs': Could not find 'bundler' (>= 0) among 146 total gem(s) (Gem::LoadError)
       +        from /Users/jesse/.rbenv/versions/2.1.9/lib/ruby/2.1.0/rubygems/dependency.rb:309:in `to_spec'
       +        from /Users/jesse/.rbenv/versions/2.1.9/lib/ruby/2.1.0/rubygems/core_ext/kernel_gem.rb:53:in `gem'
       +        from /Users/jesse/.rbenv/versions/2.1.9/bin/bundle:22:in `<main>'

Edit: Actually, looking at the stacktrace it looks like it was the bundle executable itself which couldn't find bundler.