.travis.yml: Documentation build test

Let's certify that Sphinx documentation can be properly
built as well.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
.travis.yml: Search in the output of sphinx build for errors

In order to avoid errors and garbage creeping up in the
code base, look at the sphinx build output for errors and
warnings, and fail if we found any.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #45 from avocado-framework/add-doc-build-test

.travis.yml: Documentation build test