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.

(cherry picked from commit aca929de)

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