Require automake 1.11 and parallel tests.

This seems to be the cause of the test failures on Travis CI, as it
doesn't have automake 1.13. That's the version where parallel tests
became the default and I guess the serial test driver doesn't work the
same way.
Tweak Travis script.

* Enable parallel build and testing.
* Enable output of test failure information since we can't really
  examine the log file on Travis.
Enable portability checks for automake.
Specify test extensions for automake.