Pin sphinx version in doc building

.travis.yml

@@ -58,10 +58,10 @@ env:
 
     matrix:
         # Make sure that egg_info works without dependencies
-        - PYTHON_VERSION=2.7 SETUP_CMD='egg_info' SPHINX_VERSION='<1.6'
-        - PYTHON_VERSION=3.4 SETUP_CMD='egg_info' SPHINX_VERSION='<1.6'
-        - PYTHON_VERSION=3.5 SETUP_CMD='egg_info' SPHINX_VERSION='<1.6'
-        - PYTHON_VERSION=3.6 SETUP_CMD='egg_info' SPHINX_VERSION='<1.6'
+        - PYTHON_VERSION=2.7 SETUP_CMD='egg_info'
+        - PYTHON_VERSION=3.4 SETUP_CMD='egg_info'
+        - PYTHON_VERSION=3.5 SETUP_CMD='egg_info'
+        - PYTHON_VERSION=3.6 SETUP_CMD='egg_info'
 
 matrix:
 
@@ -80,7 +80,7 @@ matrix:
         # Check for sphinx doc build warnings - we do this first because it
         # may run for a long time
         - os: linux
-          env: SETUP_CMD='build_docs -w'
+          env: SETUP_CMD='build_docs -w' SPHINX_VERSION='<1.6'
 
         # Now try Astropy dev with the latest Python and LTS with Python 2.7 and 3.x.
         - os: linux