Merge pull request #159 from rigetticomputing/fix_semaphore

Fix cvxpy version so semaphore passes

tox.ini

@@ -20,7 +20,7 @@ commands=
     pip install cython==0.24.1
     pip install qutip==4.1
     pip install scs==1.2.6
-    pip install cvxpy
+    pip install cvxpy==0.4.11
 
     # backend that works without X running
     echo "backend : Agg" > ~/.config/matplotlib/matplotlibrc