Pin the rubygems version for Ruby 2.1.9
Pin the rubygems version for Ruby 2.1.9

Prior to this commit, travis would fail to update ruby gems as version
3.x requires ruby 2.3+. This commit pins the rubygems version to 2.7.8
for ruby 2.1.9.

mkulke/hiera-eyaml,TomPoulton/hiera-eyaml,TomPoulton/hiera-eyaml,mkulke/hiera-eyaml