[CI] Add more Python versions to Travis

Now that the tests run through with Python2.7, extend the list of Python
versions to be supported.

.travis.yml

@@ -1,10 +1,10 @@
 language: python
 python:
   - "2.7"
-    #  - "3.2"
-    #  - "3.3"
-    #  - "3.4"
-    #  - "3.5"
+  - "3.2"
+  - "3.3"
+  - "3.4"
+  - "3.5"
 install:
   - pip install psutil
   - pip install netifaces