First, setup Bundler. Then, require it.

Per a conversation in
https://github.com/bundler/bundler/issues/3252#issuecomment-63208769
Only try bundler if the Gemfile is there.
Add remote-include plugin to list
Merge pull request #3136 from knorthfield/patch-1
Update history to reflect merge of #3136 [ci skip]
Keep track of the required gems.
Merge pull request #3119 from jekyll/fix-bundle-require
Update history to reflect merge of #3119 [ci skip]