fix rule names so other ANTLR4 targets work
fix rule names so other ANTLR4 targets work

First triggered by using ANTLR4 Python backend where True and False are
reserved key-words and as such the generated Python code by ANTLR4 won't
work. This solves the issue without changing the underlying DSL that is
being parsed by this ANTLR4 grammar.

tjitze/RankPL