Eliminate repetition in the testing requirements files.

requirements/testing-base.txt

@@ -0,0 +1,6 @@
+codecov==1.4.1
+coverage==3.7.1
+coveralls==0.5
+django-test-without-migrations==0.2
+tox==2.1.1
+

requirements/testing-no-django.txt

@@ -1,6 +1,3 @@
-# requirements/testing.txt
 -r base.txt
-codecov==1.4.1
+-r testing-base.txt
-coverage==3.7.1
+
-coveralls==0.5
-django-test-without-migrations==0.2

requirements/testing.txt

@@ -1,6 +1,3 @@
-# requirements/testing.txt
 -r common.txt
-codecov==1.4.1
+-r testing-base.txt
-coverage==3.7.1
+
-coveralls==0.5
-django-test-without-migrations==0.2