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 Slack build notifications.
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]
Prepare for a v2.5.2 release.
Merge pull request #3117 from jekyll/release-2-5-2
Capture the output of Jekyll::Commands::New when looking for an error.
Better tests for plugin manager.
Put that Bundle.require statement back.
Refactor the case statement in highlighter for better readability. [ci skip]
Merge pull request #3141 from jekyll/bundle-require-is-borking-pygments

Fix Bundler-integration Plugin Manager
Release :gem: 2.5.2
Docs: use font-awesome for permalink too.

Also remove the unused `permalink` class from the `a` tag.