Inconsistency with official libyara grammar

[AJohnson24]

While attempting to parse a valid yara ruleset with the latest version of master, the following error was returned by gyp.Parse(): syntax error: unexpected _HEX_NUMBER_, expecting _NUMBER_

The relevant snippet of the line in the ruleset where the error occurred is as follows: xor(0x01-0xff)

I've verified that no error is returned in v0.9.0, so I believe the bug was introduced in the latest commit, when the new _HEX_NUMBER_ and _OCT_NUMBER_ tokens introduced an inconsistency with the official libyara grammar, and the xor string modifiers definition was not updated.