requirements: Move common includes to py2/py3, not dev/prod.

This is a nonfunctional refactor of how the common.txt requirements
are included.  It is preparation for a new model for freezing our
recursive dependencies based on `pip compile`.

requirements/dev.txt

@@ -1,4 +1,3 @@
--r common.txt
 -r docs.txt
 -r moto.txt
 -r py3k.txt

requirements/prod.txt

@@ -1,4 +1,3 @@
--r common.txt
 netifaces==0.10.5
 python-dateutil==2.5.3
 uWSGI==2.0.14

requirements/py2_common.txt

@@ -1,3 +1,5 @@
+-r common.txt
+
 # Used for Hesiod lookups, etc.
 pydns==2.3.6
 

requirements/py3_common.txt

@@ -1,3 +1,5 @@
+-r common.txt
+
 # Used for Hesiod lookups, etc.
 py3dns==3.1.0