remove hard coded column references

[tzwilliams]

Some code uses numerical column references rather than column names. Find and fix these. For example,

edx-studentcourseanalytics/edX-courseStructureMeta.R

Line 126 in 2f99dfd

modList <- modList[,c(1:4,20,21)] #TODO(call these columns by name)

was updated to work with B2, but now it won't work with B1.