databases/couchdb2: bump PORTREVISION after lang/erlang-runtime21 was updated

As lang/erlang-runtime21 is a dependency, it requires a PORTREVISION
bump to ensure BEAM bytecode in CouchDB is updated accordingly. Byte
code is versioned so the module version changes in Erlang break those
in CouchDB.

Approved by: jrm (mentor)
MFH: 2018Q4
Differential Revision: https://reviews.freebsd.org/D18597