(PUP-3277) Fix acceptance Gemfile logic for choosing gemsource

Since there were no parentheses around the trinary, if GEM_SOURCE was
set, it would be immediately ||'d with OUTSIDE_PUPPETLABS, causing
rubygems.org to be selected as the path instead of GEM_SOURCE.  Broke
this bad logic up to make it explicit that GEM_SOURCE takes precedence,
and we only worry about a default if it is not set, then choosing
between two options based on whether the user has set the
OUTSIDE_PUPPETLABS flag.
Merge branch 'issue/master/pup-3277-fix-gemfile'