Gem dependency failure

[gracecodes144]

It seems like some recent gem updates have been causing errors between jekyll and mercenary versions. I'm not exactly sure what the problem is, but found these issues: 1 and 2

My docker logs jekyll_container output looked like:

/usr/local/bundle/gems/jekyll-2.5.3/bin/jekyll:21:in `block in <top (required)>': cannot load such file -- jekyll/version (LoadError)
    from /usr/local/bundle/gems/mercenary-0.3.5/lib/mercenary.rb:18:in `program'
    from /usr/local/bundle/gems/jekyll-2.5.3/bin/jekyll:20:in `<top (required)>'
    from /usr/local/bundle/bin/jekyll:23:in `load'
    from /usr/local/bundle/bin/jekyll:23:in `<main>'

The tickets I found all suggest using bundle exec jekyll command as a fix. I'll submit a PR with this change to the entrypoint shortly.