travis: use mako for python2

Earlier commit flipped the default to python2 but forgot to update the
travis file. Props to pip caching things "worked" for a little while.

Fixes: f22ad5ef18 ("travis: use python3 for the autoconf builds")
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>

.travis.yml

@@ -541,7 +541,7 @@ install:
 
   # Install autotools build dependencies
   - if test "x$BUILD" = xmake; then
-      pip3 install --user mako;
+      pip2 install --user mako;
     fi
 
   # Install a more modern scons from pip.