Rename ActiveTable max_score to maximum_score to avoid conflicts.

See https://openedx.atlassian.net/browse/EDUCATOR-338
We defined max_score as a Float XBlock field, while
a method max_score exists that was getting conflicts
with it.

requirements/edx/edx-private.txt

@@ -34,4 +34,4 @@ ubcpi-xblock==0.6.2
 
 # Vector Drawing and ActiveTable XBlocks (Davidson)
 -e git+https://github.com/open-craft/xblock-vectordraw.git@v0.2.1#egg=xblock-vectordraw==0.2.1
--e git+https://github.com/open-craft/xblock-activetable.git@1786a2c8fc06ca3ecfac1972464048982bd486ff#egg=xblock-activetable
+-e git+https://github.com/open-craft/xblock-activetable.git@e933d41bb86a8d50fb878787ca680165a092a6d5#egg=xblock-activetable