Error:
/home/.../ruby/gems/2.1.0/gems/bundler-1.14.3/lib/bundler/rubygems_ext.rb:45:in `full_gem_path': uninitialized constant Bundler::Plugin::API::Source (NameError)
Rails LTS provides security patches for old versions of Ruby on Rails (2.3, 3.2, 4.2 and 5.2)
Solution: Upgrade Rubygems beyond 2.6.9.