Pin commonmark to 0.5.5 in the docs too

0.6 changes API and breaks things so make sure users are installing
the right version.

Author: LinuxJedi

README.rst

@@ -31,7 +31,7 @@ There are a couple of extra dependencies you will also need to build the Markdow
 
 .. code-block:: bash
 
-   $ sudo pip install recommonmark commonmark
+   $ sudo pip install recommonmark commonmark==0.5.5
 
 You can then build the docs with:
 

source/contributing/github.rst

@@ -21,7 +21,7 @@ You also need *python-sphinx* installed; some Linux distributions have this in t
 
 .. code-block:: bash
 
-   $ pip install -U Sphinx recommonmark commonmark
+   $ pip install -U Sphinx recommonmark commonmark==0.5.5
 
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