Update Python versions for Travis CI tests

Author: kirsle

.travis.yml

@@ -1,11 +1,10 @@
 language: python
 python:
   - "2.7"
-  - "3.2"
-  - "3.3"
   - "3.4"
   - "3.5"
   - "3.6"
-  - "nightly"
+  - "3.7"
+  - "3.8"
 install: "pip install -r requirements.txt"
 script: nosetests