add python osx 2.7 to travis testing

.travis.yml

@@ -1,27 +1,43 @@
-os:
-  - linux
-language: python
-python:
-  - "2.7"
-  - "3.3"
-  - "3.4"
-  - "3.5"
-  - "3.6"
+# list of environments to test
 matrix:
   include:
-    - language: generic
-      os: osx
-      python:
-        - "3"
+    - os: linux
+      language: python
+      python: 2.7
+    - os: linux
+      language: python
+      python: 3.3
+    - os: linux
+      language: python
+      python: 3.4
+    - os: linux
+      language: python
+      python: 3.5
+    - os: linux
+      language: python
+      python: 3.6
+    - os: osx
+      language: generic
+      python: 2.7
+      before_install:
+        - python2 --version
+        - pip2 install -U virtualenv
+        - virtualenv env -p python2
+        - source env/bin/activate
+    - os: osx
+      language: generic
+      python: ">=3.6"
       before_install:
         - brew update
         - brew outdated python3 || brew install python3 || brew upgrade python3
-        - pip install -U virtualenv
-        - virtualenv env -p python3
+        - python3 -m venv env
         - source env/bin/activate
+
 # command to install dependencies
 install:
-    - "pip install -r tests/requirements.txt"
-    - "python setup.py install"
+    - python --version
+    - pip install -r tests/requirements.txt
+    - python setup.py install
+
 # command to run tests
 script: python -m unittest discover