(maint) Update Gemfile with GEM_FACTER_VERSION

(maint) Update Gemfile with GEM_FACTER_VERSION

Without this patch we cannot explicitly set the version of Facter to
integrate using Bundler.  This patch addresses the problem by adding a
new environment variable, GEM_FACTER_VERSION which allows bundler to
install a specific version of Facter.  GEM_FACTER_VERSION is the
variable name instead of FACTER_GEM_VERSION to prevent the gem_version
fact from being defined.  In addition, GEM_PUPPET_VERSION is defined
based on PUPPET_GEM_VERSION in order to match the environment names and
provide backwards compatibility with CI jobs.