Fix jenkins CI complaining of comments in Gemfile

Lifted from CirrusSearch.
Avoids (non-voting) failure in mwext-Flow-bundle-rubocop:
  02:52:14 Gemfile:1:1: C: Missing space after #.
  02:52:14 #ruby=ruby-2.1.1

Change-Id: I2e5c38047e5fc402e746c58c09b047d0e8e7ee3b