Modify .travis.yml to do the doc build

Also:
- Have doc build treat warnings as errors

Author: tleonhardt

.travis.yml

@@ -16,6 +16,9 @@ matrix:
         - os: linux
           python: 3.7-dev
           env: TOXENV=py37
+        - os: linux
+          python: 3.5
+          env: TOXENV=docs
 #        # Warning: Don't try to use code coverage analysis with pypy as it is insanely slow
 #        - os: linux
 #          python: pypy3

tox.ini

@@ -10,12 +10,12 @@ setenv =
     PYTHONPATH={toxinidir}
 
 [testenv:docs]
-basepython = python3.7
+basepython = python3.5
 deps =
   sphinx
   sphinx-rtd-theme
 changedir = docs
-commands = sphinx-build -a -b html -d {envtmpdir}/doctrees . {envtmpdir}/html
+commands = sphinx-build -a -W -T -b html -d {envtmpdir}/doctrees . {envtmpdir}/html
 
 [testenv:py34]
 deps =