Simplify tox.ini

Use the factor-conditional settings feature of tox >= 1.8 to reduce
duplication:
https://tox.readthedocs.org/en/latest/config.html#factors-and-factor-conditional-settings

tox.ini

@@ -2,25 +2,12 @@
 envlist = py26,py27,py34
 
 [testenv]
-commands = make tests
 deps = -r{toxinidir}/test-requirements.txt
 whitelist_externals = make
-
-[testenv:py26]
-commands =
-    python --version
-    python -m compileall -fq -x 'test|samples|contrib/inventory/vagrant.py' lib test contrib
-    make tests
-
-[testenv:py27]
-commands =
-    python --version
-    python -m compileall -fq -x 'test|samples' lib test contrib
-    make tests
-
-[testenv:py34]
 commands =
     python --version
-    python -m compileall -fq -x 'lib/ansible/module_utils|lib/ansible/modules' lib test contrib
+    py26: python -m compileall -fq -x 'test|samples|contrib/inventory/vagrant.py' lib test contrib
+    py27: python -m compileall -fq -x 'test|samples' lib test contrib
+    py34: python -m compileall -fq -x 'lib/ansible/module_utils|lib/ansible/modules' lib test contrib
     # Unittests need lots of work to make code python3 compatible
-    #make tests
+    py{26,27}: make tests