Dropped testing for Python 3.6

Testing within Travis CI with Python 3.6 results in error:

"Direct url requirement (like vtr_xml_utils@
git+https://github.com/Symbiflow/vtr-xml-utils) are not allowed for
dependencies"

In Python 3.5 and 3.7 the error does not occur in Travis CI.

Signed-off-by: Grzegorz Latosinski <glatosinski@antmicro.com>