CI: bump minimum Sphinx version tested from 1.2.3 to 1.6.5

1.6.5 is now 1.5 years old, should be enough.

sphinx.util.logging is not present in 1.2.3, was introduced in 1.6.0;

Author: rgommers

.travis.yml

@@ -6,7 +6,7 @@ sudo: false
 matrix:
   include:
     - python: 3.7
-      env: SPHINX_SPEC="==1.2.3" SPHINXOPTS=""
+      env: SPHINX_SPEC="==1.6.5" SPHINXOPTS=""
     - python: 3.7
     - python: 2.7
 cache: