Update travis to test against different versions of sphinx

Author: djungelorm

.travis.yml

@@ -4,8 +4,15 @@ python:
   - "3.4"
   - "3.5"
   - "3.6"
+env:
+  - SPHINX_VERSION=">=1.2, <1.3"
+  - SPHINX_VERSION=">=1.3, <1.4"
+  - SPHINX_VERSION=">=1.4, <1.5"
+  - SPHINX_VERSION=">=1.5, <1.6"
 
-install: "pip install -r requirements.txt"
+install:
+ - pip install "sphinx $SPHINX_VERSION"
+ - pip install -r requirements.txt
 
 script:
  - python setup.py install