move to multiple python versions in travis

Author: Israel Fruchter (ifruchte)

.travis.yml

@@ -1,4 +1,10 @@
 language: python
+python:
+  - "2.7"
+  - "3.4"
+  - "3.5"
+  - "3.6"
+  - "nightly"
 # command to install dependencies
 install:
   - pip install python-coveralls virtualenv