sysutils/rubygem-bosh_cli: Make version requirement to json_pure less strict

With the recent updates to the json_pure gem, the version requirement
in bosh_cli is too strict. After using it for a day, it looks like the
latest json_pure is compatible with bosh_cli. Thus, make it less strict.

Bump PORTREVISION.

Approved by: antoine (mentor)
Differential Revision: https://reviews.freebsd.org/D9156