Fix test that fails in docker

This test fails because it's not using File.join on ENV["HOME"] (which
is null when run under root in docker) which means that we're asserting
on the direction "//test_history" instead of "/test_history".
Add scripts to `rake test` on all rubies

Run ./multi_test.sh to test all rubies that are configured in
Dockerfile.
Add rake task for running tests on multiple rubies
Merge pull request #1268 from AndrewVos/multi-test

Easily `rake test` in a whole load of rubies