(PUP-2339) Modify $GEM_SOURCE for CI bootstrap

It is preferable to override the authoritative source (rubygems.org)
with our own internal resources, as opposed to the other way around.
This change ensures that if $GEM_SOURCE is set in the environment,
it is honored. Otherwise, we fall back to the public authority,
rubygems.org.

This change is analogous to the following line in our Gemfiles:

    source ENV['GEM_SOURCE'] || "https://rubygems.org"
Merge branch 'stable'
(maint) Update beaker to 1.11.0

With beaker 1.10.0 (and probably any earlier version that included the
dependency on google-api-client), we started seeing this error:

/usr/local/opt/rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/bundler-1.5.2/lib/bundler/rubygems_integration.rb:250:in `block in replace_gem': can't activate jwt (~> 0.1.4), already activated jwt-1.0.0.

This issue is discussed in https://tickets.puppetlabs.com/browse/QENG-655,
and it appears to be addressed by beaker 1.11.0.
Merge branch 'stable'