travis: use tests/requirements.txt

travis: use tests/requirements.txt

Explicitly install ansible-lint pytest pytest-cov via pip results of a
specific pytest version (4.3.1) which is not supported for pytest-cov
(2.10).
Because we are already defining a specific pytest version in the tests
requirements then we can install all the python dependencies from that
file and remove this from the pip install command.

Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>