ruby: The `/=` operator doesn't need augmented priority

Unless I am mistaken, this was only relevant before the regexp matches had heuristics tor precedence. Now it causes weird behavior.

Removing this augmented priority fixes many issues with regexes that begin with `/=`.

For instance, the following expression in a switch statement would fail to be highlighted before this patch: `case /=/\n`, and instead be interpreted as a `/=` and the start of a multi-line regexp.