Make tests return non-zero status on failure

All the test were being run properly, and failures could be locally
observed by reading console output. However, the travis-ci script was
assuming that the tests would return a non-zero status code on failure,
which was not happening.

In order to correct this, the following line was added to the bottom of
tests/Test.ml:

```
  Pa_ounit_lib.Runtime.summarize ()
```

As the name implies, this will summarize the results of the test run and
in addition return a non-zero status if any tests failed.