Fix for issue #4

Looks like the `ConcurrentModificationException` we were seeing in the tests running on travis is a bug [SI-7775](https://issues.scala-lang.org/browse/SI-7775) in scala.

I have no idea why this only happens on travis and not locally, but running the travis job with `2.10.3-RC2` (which contains the fix for SI-7775) means the test suite runs on travis with no problems.

All tests passing: https://travis-ci.org/SCCT/scct/builds/11940789