Fix 'docs' tox environment and change warnings into errors

Also runs the 'docs' job on Travis to ensure documentation is building
correctly.

.travis.yml

@@ -18,6 +18,8 @@ jobs:
       python: '3.5'
     - env: TOXENV=py36
       python: '3.6'
+    - env: TOXENV=docs
+      python: '3.6'
     - env: TOXENV=py37
       python: '3.7'
       sudo: required

docs/conf.py

@@ -132,7 +132,7 @@
 # Add any paths that contain custom static files (such as style sheets) here,
 # relative to this directory. They are copied after the builtin static files,
 # so a file named "default.css" will overwrite the builtin "default.css".
-html_static_path = ['_static']
+#html_static_path = ['_static']
 
 # Add any extra paths that contain custom files (such as robots.txt or
 # .htaccess) here, relative to this directory. These files are copied

docs/features.rst

@@ -230,20 +230,20 @@ provides an easy way to test content negotiation in your application:
 
 
 ``accept_any`` - :mimetype:`*/*` accept header
-""""""""""""""""""""""""""""""""""""""""""""""
+``````````````````````````````````````````````
 
 :mimetype:`*/*` accept header suitable to use as parameter in ``client``.
 
 
 ``accept_json`` - :mimetype:`application/json` accept header
-""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
+````````````````````````````````````````````````````````````
 
 :mimetype:`application/json` accept header suitable to use as parameter in
 ``client``.
 
 
 ``accept_jsonp`` - :mimetype:`application/json-p` accept header
-"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
+```````````````````````````````````````````````````````````````
 
 :mimetype:`application/json-p` accept header suitable to use as parameter in
 ``client``.

tox.ini

@@ -31,10 +31,8 @@ commands =
 
 [testenv:docs]
 changedir = docs
-deps = -r../requirements/docs.txt
-
+skipsdist = True
+usedevelop = True
+deps = -r requirements/docs.txt
 commands =
-    make html
-
-whitelist_externals =
-    /usr/bin/make
+    sphinx-build -W -b html . _build