Test that optional dependencies are loaded correctly

[skalee]

A bug fixed in #130 was a result of lack of tests which cover load order. This issue expresses need for such tests.

It will be very difficult to do such tests in RSpec — our test suite needs these optional dependencies in advance during test setup. I suppose the only reasonable solution is to write some standalone executable examples, similar to these from https://github.com/riboseinc/activeuuid/tree/master/examples. Nevertheless, I haven't thought it thoroughly yet.