update .travis.yml to use a matrix of versions in build

.travis.yml

@@ -1,8 +1,16 @@
 language: python
 sudo: false
 cache: pip
+matrix:
+  include:
+  - python: 2.7
+    env: TOXENV=py27
+  - python: 3.4
+    env: TOXENV=py34
+  - python: 3.5
+    env: TOXENV=py35
 before_install:
     - pip install tox
 # command to run tests
 script:
-    - tox
+    - tox