Reorganize requirements.txt and Travis CI job

Document requirements.txt, split it into 3 files:

 * requirements.txt - Used to install deps for functionality
 * tests-requirements.txt - Used to install deps to test avocado
 * travis-requirements.txt - Used exclusively for Travis CI jobs

Also, clean up and simplify Travis install job section.

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