Test with Python 3.6 (#102)

diff --git a/tox.ini b/tox.ini
index a2f7362..528806c 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,5 +1,5 @@
 [tox]
-envlist = lint,py27,py34,py35,pypy,cover
+envlist = lint,py27,py34,py35,py36,pypy,cover
 
 [testenv]
 deps =
@@ -18,15 +18,15 @@
   py.test --cov=google.auth --cov=google.oauth2 --cov=tests {posargs:tests}
 
 [testenv:cover]
-basepython = python3.5
+basepython = python3.6
 commands =
   py.test --cov=google.auth --cov=google.oauth2 --cov=tests --cov-report= tests
   coverage report --show-missing --fail-under=100
 deps =
   {[testenv]deps}
 
-[testenv:py35-system]
-basepython = python3.5
+[testenv:py36-system]
+basepython = python3.6
 changedir = {toxinidir}/system_tests
 commands =
   nox {posargs}
@@ -52,7 +52,7 @@
   CLOUD_SDK_ROOT
 
 [testenv:docgen]
-basepython = python3.5
+basepython = python3.6
 deps =
   {[testenv]deps}
   sphinx
@@ -63,7 +63,7 @@
   sphinx-apidoc --output-dir docs/reference --separate --module-first google
 
 [testenv:docs]
-basepython = python3.5
+basepython = python3.6
 deps =
   sphinx
   -r{toxinidir}/docs/requirements-docs.txt