Move linkcheck tox env to use setup.py to get docs dependencies (#4473)

diff --git a/tox.ini b/tox.ini
index a6227c0..c313e9c 100644
--- a/tox.ini
+++ b/tox.ini
@@ -50,8 +50,7 @@
 
 [testenv:docs-linkcheck]
 extras =
-deps =
-    sphinx
+    docs
 basepython = python2.7
 commands =
     sphinx-build -W -b linkcheck docs docs/_build/html