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.