Use python version-specific build distros

Author: tboggs

.travis.yml

@@ -1,12 +1,14 @@
 language: python
 sudo: false
-python:
-  - "2.6"
-  - "2.7"
-  - "3.3"
-  - "3.5"
-  - "3.6"
-install:
+python: ["2.7", "3.5", "3.6", "3.7"]
+matrix:
+  include:
+    - python: 2.6
+      dist: trusty
+    - python: 3.3
+      dist: trusty
+
+  install:
   - pip install numpy
   - python setup.py install
 before_script: