Annotate failing 2.6 tests

Disable only the tests that fail on 2.6. Consequently, re-enable testing on Travis. With this, I can [build packages](https://copr.fedoraproject.org/coprs/qulogic/obspy/monitor/) from EPEL 6 to Fedora Rawhide with the full(-ish) set of tests passing. Hopefully, this will also pass on Travis as well.

Again, I only marked things with the status quo.