CI: bump minimum Sphinx version tested from 1.2.3 to 1.5.6

1.5.6 is now 2 years old, should be enough.

sphinx.util.logging is not present in 1.2.3, was introduced in 1.4.0;
1.4.x still doesn't have getLogger() so trying 1.5.x

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="==1.5.6" SPHINXOPTS=""
     - python: 3.7
     - python: 2.7
 cache: