.travis.yml: Remove unneeded deps, add flexmock

It'll be necessary for the data_dir unittest.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
unittests: Add datadir_unittest

Add one meager unittest that verifies part of the intended
behavior for the data_dir module.

This unittest requires flexmock, that was added to the
travis job in the previous commit.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
docs: Add data_dir documentation

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>