Build docs in parallel (#1842)

.travis.yml

@@ -42,7 +42,7 @@ 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"
     - os: linux
       language: c
       compiler: gcc