Travis-ci updates and repairs

Travis-ci updates and repairs

Since travis does a shallow clone by default, a problem can emerge when the
last tag is outside the clone depth. In such cases, setup.py cannot
determine the stgit version and silently fails to generate the
builtin_version.py file.

The shallow clone problem is solved by conditionally unshallowing the repo
if no tags are found.

Another problem is that coveralls is not available for Python 2.6. This
problem is solved by conditionally installing coveralls only for the
'coverage-test' build targets.

The travis configuration is also modified to remove the 'sudo:' config
option. Using `sudo: false` caused the builds to be run in a containerized
environment whereas `sudo: true` caused a virtual machine environment to be
used. Removing `sudo:` altogether indicates to Travis that the stgit build
can use their default environment which is transitioning to VM's.

Lastly, instead of specifying `dist: trusty` the config now specifies
`group: travis_latest`. In the immediate term, the builds will still use
the trusty (Ubuntu 14.04) environment since that is still Travis' latest,
but future builds will use newer environments as Travis makes them
available.

Signed-off-by: Peter Grayson <pete@jpgrayson.net>