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>

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index b44ffeb..eec58ba 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,8 +2,6 @@
 
 matrix:
   include:
-    - python: 3.6
-      env: TOXENV=py36
     - python: 3.7
       env: TOXENV=py37