Move matplotlib to post requirements.

Matplotlib depends on numpy, so we want to ensure the correct version
of numpy is already installed when we attempt to install matplotlib.

requirements/edx-sandbox/base.txt

@@ -13,7 +13,6 @@ numpy==1.6.2
 networkx==1.7
 sympy==0.7.1
 pyparsing==2.0.7
-matplotlib==1.3.1
 
 # We forked NLTK just to make it work with setuptools instead of distribute
 git+https://github.com/edx/nltk.git@2.0.6#egg=nltk==2.0.6

requirements/edx-sandbox/post.txt

@@ -7,3 +7,4 @@
 # Packages to install in the Python sandbox for secured execution.
 scipy==0.14.0
 lxml==3.4.4
+matplotlib==1.3.1