RSpec 3 support (& fix tests under rbx-2)

1. Upgrade rspec dependency to `~> 3.0`
2. Move config from `~/.rspec` into `spec/spec_helper.rb`
3. Enable "legacy" `should` syntax as well as new `expect` syntax for now
4. Update specs where we have to - `be_true` and `be_false` no longer work as expected
5. Test suite runs under rbx-2 again locally - it didn't like rspec 2.99.0 for some reason