maint: don’t use \] in BREs and EREs

* .travis.yml (jobs):
* doc/local.mk ($(top_srcdir)/%D%/bison.help):
* tests/regression.at (Web2c Actions, Useless Tokens):
* tests/scanner.at (Token numbers: $1):
Use plain ], not \], in a POSIX regular expression,
as POSIX says the interpretation of \] is undefined.