Support installing extra gems per module

A handful of modules require random gems. e.g webmock. Rather than installing webmock for all builds the following addition to the .sync.yml will:

• Add 'webmock' to the list of gems in Gemfile.
• Remove the --local from install so internet will be contacted to complete the gem installation.

ci/.gitlab-ci.yml:
  localonly: false

ci/Gemfile:
  optional:
    ':test':
      - gem: webmock

I've added custom as example module in the CI which now uses this mechanism. The addition of the webmock gem will be tested by this well but the localonly part will not but that's some what trivial anyway.