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.

doc/local.mk

@@ -114,7 +114,7 @@ $(top_srcdir)/%D%/bison.help: src/bison$(EXEEXT)
 	$(AM_V_at) LC_ALL=C tests/bison --version >%D%/bison.help.tmp
 	$(AM_V_at) LC_ALL=C tests/bison --help | \
 ## Avoid depending on the path to Bison.
-	  sed -e 's,^Usage: .*/bison \[OPTION\],Usage: bison [OPTION],g' \
+	  sed -e 's,^Usage: .*/bison \[OPTION],Usage: bison [OPTION],g' \
 ## Avoid variations in the output depending on whether we are
 ## on a glibc system.
 	      -e '/translation bugs/d'  >>%D%/bison.help.tmp