[6.0] 18920d641 Also install python-sphinx

[Federico G. Schwindt]

commit 18920d641c6cdf5c401f7084db13575d289aad02
Author: Federico G. Schwindt <fgsch at lodoss.net>
Date:   Tue Apr 3 10:46:47 2018 +0100

    Also install python-sphinx
    
    Required for make distcheck.
    
    Spotted by geoff.

diff --git a/.travis.yml b/.travis.yml
index 9eb8e4a39..858d0799f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -23,6 +23,7 @@ addons:
   apt:
     packages:
       - python-docutils
+      - python-sphinx
 notifications:
   irc:
     channels: