Django 1.5 support: ExpressionNode's AND/OR are now BITAND/BITOR

As mentioned in the [1.5 release notes](https://docs.djangoproject.com/en/dev/releases/1.5/#miscellaneous):

> The F() expressions supported bitwise operators by & and |. These operators are now available using .bitand() and .bitor() instead. The removal of & and | was done to be consistent with Q() expressions and QuerySet combining where the operators are used as boolean AND and OR operators.

Along with this change, `ExpressionNode.AND` has become `ExpressionNode.BITAND` and `ExpressionNode.OR` has become `ExpressionNode.BITOR`, breaking `sentry/utils/db.py`.