Update travis configuration. (#63)

Author: Adminiuga

.travis.yml

@@ -2,12 +2,14 @@ language: python
 matrix:
   fast_finish: true
   include:
-    - python: "3.5"
+    - python: "3.6"
       env: TOXENV=lint
-    - python: "3.5"
-      env: TOXENV=py35
     - python: "3.6"
       env: TOXENV=py36
+    - python: "3.7"
+      env: TOXENV=py37
+    - python: "3.8"
+      env: TOXENV=py38
 install: pip install -U setuptools tox coveralls
 script: tox
 after_success: coveralls