TST: Add Sphinx 1.6.6 to the versions to be tested

Author: mgeier

.travis.yml

@@ -15,6 +15,7 @@ matrix:
   - env: SPHINX="==1.5.1"
   - env: SPHINX="==1.5.3"
   - env: SPHINX="==1.5.6"
+  - env: SPHINX="==1.6.6"
 
   # a few Python versions using latest Sphinx release
   - python: "3.4"