Bug 1489407 - Docs: Pin to older version of CommonMark to fix build

CommonMark has made a breaking package change in a minor version
release (0.8.0), so we need to pin to the last working release. See:
readthedocs/commonmark.py#134

Fixes:

```
  ...
  File ".../recommonmark/recommonmark/parser.py", line 9, in <module>
    from CommonMark import Parser
ImportError: No module named CommonMark
```

The Travis docs setup step has also been modified to always upgrade
dependencies, which will mean we will catch cases like this on Travis
next time.

Author: edmorley

bin/travis-setup.sh

@@ -55,7 +55,7 @@ setup_python_env() {
 }
 
 setup_docs() {
-    pip install -r requirements/docs.txt
+    pip install -U -r requirements/docs.txt
 }
 
 setup_geckodriver() {

requirements/docs.txt

@@ -13,3 +13,4 @@ sphinx-rtd-theme==0.4.1
 # installed, since Read The Docs doesn't use --upgrade and has an
 # older incompatible version of recommonmark pre-installed.
 -e git+git://github.com/rtfd/recommonmark.git#egg=recommonmark
+commonmark==0.7.5