update requirements.txt to match readthedocs versions

docs/conf.py

@@ -83,7 +83,11 @@ html_theme = 'sphinx_rtd_theme'
 # further.  For a list of options available for each theme, see the
 # documentation.
 #
-# html_theme_options = {}
+html_theme_options = {
+#    'collapse_navigation': True,
+#    'display_version': True,
+#    'navigation_depth': 4,
+}
 
 # 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,

docs/requirements.txt

@@ -1,4 +1,4 @@
-sphinx
+Sphinx==1.7.4
+Pygments==2.2.0
 sphinx-autobuild
-recommonmark
-sphinx_rtd_theme
+sphinx_rtd_theme==0.2.5b1