Stub the test ruby instead of downloading a live-ruby

[postmodern]

The stub_bin_ruby branch which I have started now generates the test ruby's bin/ruby script to be a stub script that returns the expected output when the expected input is given. How or when the script should be generated, or if the bin/ruby script should be committed, are still unanswered questions.

Benefits

• No longer relying on RVM's outdated binaries.
• CI now passes!
• Speeds up CI run time.
• Saves GitHub some bandwidth.

Disadvantages

• We're essentially testing against mocks and stubs, not real-world rubies.
• Will need to add proper integration tests that run against /opt/rubies/.

[postmodern]

I am now leaning towards adding a test/configure script that will generate the test/root/opt/rubies/ directory and bin/ruby, as well as a ruby.sh helper. This can also be used to re-generate the test fixtures when we wish to change settings.

[postmodern]

Renamed test/setup to test/configure and store test configuration in test/config.sh. Now test/fixtures/root/opt/rubies are auto-re-generated on each test run.