Improve tox Django dependency declaration.

Author: cyberdelia

tox.ini

@@ -51,15 +51,15 @@ deps =
 [testenv:py27]
 basepython = python2.7
 deps =
-    Django>=1.7.0
+    Django>=1.7, <1.8
     mock
     jinja2
     futures
 
 [testenv:pypy]
 basepython = pypy
 deps =
-    Django>=1.7.0
+    Django>=1.7, <1.8
     mock
     jinja2
     futures
@@ -68,13 +68,13 @@ deps =
 basepython = python3.3
 deps =
     jinja2
-    Django>=1.7.0
+    Django>=1.7, <1.8
 
 [testenv:py34]
 basepython = python3.4
 deps =
     jinja2
-    Django>=1.7.0
+    Django>=1.7, <1.8
 
 [testenv:docs]
 basepython = python2.7