Travis fails in doc build

Travis has started failing in the documentation build, e.g.:

+cd doc-src
+make
mkdir -p _build/doctrees
sphinx-build -W -b html -D release="$(cat ../VERSION)" -D version="$(git rev-parse --short HEAD)" -d _build/doctrees   . _build/html
Running Sphinx v1.8.0
Exception occurred:
  File "/usr/local/lib/python2.7/dist-packages/sphinx/highlighting.py", line 26, in <module>
    from sphinx.ext import doctest
SyntaxError: unqualified exec is not allowed in function 'run' it contains a nested function with free variables (doctest.py, line 97)
The full traceback has been saved in /tmp/sphinx-err-9hZLhN.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!
make: *** [html] Error 2

I suspect that Sphinx v1.8.0 just came out and is buggy.