Revert "docs: Update dependencies for docs."

This reverts commit 4303b42abd.

This appears to have broken various documentation links to
e.g. README.md on GitHub.

requirements/dev.txt

@@ -37,7 +37,7 @@ certifi==2018.11.29
 cffi==1.12.2
 chardet==3.0.4
 click==6.6                # via gitlint, pip-tools
-commonmark==0.8.1
+commonmark==0.5.4
 constantly==15.1.0        # via twisted
 coverage==4.5.2
 cryptography==2.6.1
@@ -149,7 +149,7 @@ pytz==2018.9
 pyyaml==3.13              # via pyaml, yamole
 qrcode==6.1               # via django-two-factor-auth
 queuelib==1.5.0           # via scrapy
-recommonmark==0.5.0
+recommonmark==0.4.0
 redis==2.10.6
 regex==2019.2.21
 requests-oauthlib==1.0.0

requirements/docs.in

@@ -12,12 +12,15 @@ sphinx==1.8.4
 sphinx-rtd-theme==0.4.3
 
 # Needed to build markdown docs
-recommonmark==0.5.0
+recommonmark==0.4.0
 
 # Dependencies of Sphinx
 alabaster==0.7.12
 babel==2.5.3
-CommonMark==0.8.1
+# Upgrading to the latest version of CommonMark breaks the
+# compatibility with recommonmark.  See
+# https://github.com/rtfd/recommonmark/issues/24
+CommonMark==0.5.4
 docutils==0.14
 imagesize==1.1.0
 snowballstemmer==1.2.1

requirements/docs.txt

@@ -11,7 +11,7 @@ alabaster==0.7.12
 babel==2.5.3
 certifi==2018.11.29       # via requests
 chardet==3.0.4            # via requests
-commonmark==0.8.1
+commonmark==0.5.4
 docutils==0.14
 idna==2.8                 # via requests
 imagesize==1.1.0
@@ -21,7 +21,7 @@ packaging==19.0           # via sphinx
 pygments==2.3.1           # via sphinx
 pyparsing==2.3.1          # via packaging
 pytz==2018.9              # via babel
-recommonmark==0.5.0
+recommonmark==0.4.0
 requests==2.21.0          # via sphinx
 six==1.12.0               # via packaging, sphinx
 snowballstemmer==1.2.1