Install a compiler for the CI

1 job for develop in 1 minute and 13 seconds (queued for 1 second)
Status Job ID Name Coverage
  Deploy
failed #996
pages

00:01:13

 
Name Stage Failure
failed
pages Deploy
Make sure that `gem install eventmachine -v '1.2.7' --source
'http://rubygems.org/'` succeeds before bundling.

In Gemfile:
jekyll-sitemap was resolved to 1.3.1, which depends on
jekyll was resolved to 3.8.5, which depends on
em-websocket was resolved to 0.5.1, which depends on
eventmachine
ERROR: Job failed: exit code 5