Fix PySpark unit tests on Python 2.6.
Rename `memoryBytesToString` and `memoryMegabytesToString`

These are used all over the place now and they are not specific to memory at all.

memoryBytesToString --> bytesToString
memoryMegabytesToString --> megabytesToString
Merge pull request #829 from JoshRosen/pyspark-unit-tests-python-2.6

Fix PySpark unit tests on Python 2.6
Merge pull request #836 from pwendell/rename

Rename `memoryBytesToString` and `memoryMegabytesToString`