use specs2's built-in equality matcher instead of ours

In `StreamTTest`, the random inputs to the `mapM` property can produce very
long streams. When specs2 is trying to generate the `okMessage` of the
`MatchResult`, it uses `Show[Stream[Int]]`, which internally constructs a
`FingerTree`. That process causes a stack overflow.
Revert "Travis: allow build failure for 2.9.3 on openjdk6"

These failures were completely spurious.

This reverts commit 272a14a15cdb81c1f9aa6ec194f7f05e58e90277.