Today I ran into this:
Invalid gemspec in [/usr/local/rvm/gems/ruby-1.9.3-p194/specifications/ZenTest-4.9.3.gemspec]: Illformed requirement ["< 2.1, >= 1.8"].
Rails LTS provides security patches for old versions of Ruby on Rails (2.3, 3.2, 4.2 and 5.2)
You need a newer Rubygems version. Try this: gem update --system 1.8.29
gem update --system 1.8.29