Also, make the intent of the env variable clearer in travis configuration.

Fixes sbt#2357.

This is a fixup of 0f61629.
That commit assumed that dealiasing is being done for types referred in
self type. It was changed to not do that but the test wasn't updated.
Unfortunately, that mistake slipped by during PR review because unit tests
of compileInterface were not ran (see sbt#2358).