Update .travis.yml (#86)

* Update .travis.yml

add newer python versions and updated mpc dependency

Author: wilko77

.travis.yml

@@ -7,7 +7,9 @@ python:
   - '3.4'
   - '3.5'
   - '3.6'
-  - '3.7-dev'
+  - '3.7'
+  - '3.8'
+  - '3.9-dev'
   - 'nightly'
   - 'pypy3'