Removing a superfluous cross-build to reduce chance of spurious CI death

...because this happened:

https://travis-ci.org/rtyley/bfg-repo-cleaner/builds/25364215
Move massive-non-file-objects test to separate class...

JGit has this rather awful JVM-wide static config for WindowCache, which
means that trying to run lots of concurrent test with different
configuration values is doomed to failure:

http://dev.eclipse.org/mhonarc/lists/jgit-dev/msg02530.html

...I had a go at getting forking applied for *just-this-test*.

http://www.scala-sbt.org/0.13.2/docs/Detailed-Topics/Testing#forking-tests

...but couldn't satisfy myself that I was actually getting the behaviour
I wanted, so have let it go for the time being.