Fix test incompatibility with Python 3.7 and bump travis versions

[9thbit]

There was a trailing comma in the repr of ValueError exceptions prior to python 3.7. The removal of that is breaking tests under 3.7, so this updates the expected string to incorporate whatever the built-in repr for ValueError is.

Also bumps the python version in the travis config to the official 3.7 release, rather than dev, and disallows failures.

[9thbit]

My apologies for the travis build failures, I'm not too familiar with it. There seem to be odd limitations around specifying the more recent python versions. I bumped the distro version to xenial in the config, but the oldest python versions are failing now. I guess it raises the question if you would like to continue supporting these older releases, specifically 2.6 and 3.3? Happy to take your direction on this or chat. :)