Added Sphinx 1.8.0 support for the testing on Travis-CI

requirements.txt

@@ -0,0 +1,2 @@
+# sphinx 1.8.0
+-e git+https://github.com/sphinx-doc/sphinx@master#egg=sphinx

tox.ini

@@ -12,12 +12,14 @@ setev =
     LANG=C
 deps =
     .
+    -r{toxinidir}/requirements.txt
     readthedocs-sphinx-ext
     pytest
     sphinx13: Sphinx < 1.4
     sphinx14: Sphinx < 1.5
     sphinx15: Sphinx < 1.6
     sphinx16: Sphinx < 1.7
+    sphinx17: Sphinx < 1.8
 commands =
     py.test {posargs}