[2.7] Build docs in parallel (GH-1842). (GH-1873)

(cherry picked from commit eba68e2c)

.travis.yml

@@ -32,7 +32,7 @@ matrix:
         # (Updating the version is fine as long as no warnings are raised by doing so.)
         - python -m pip install sphinx~=1.6.1
       script:
-        - make check suspicious html SPHINXOPTS="-q -W"
+        - make check suspicious html SPHINXOPTS="-q -W -j4"
 
 # Travis provides only 2 cores, so don't overdue the parallelism and waste memory.
 before_script: