lang/php56: Change from oniguruma5 to oniguruma6

Switch mbstring extension from devel/oniguruma5
to devel/oniguruma6 to fix security issues covert in
Oniguruma 6.4.

PR: 220809
Submitted by: Dan Langille <dvl@FreeBSD.org>
Approved by: maintainer (timeout, 16 days)
MFH: 2017Q3