MFH: r457878 r457976

Fix build on Ruby prior to 2.5

Ruby 2.5 bundled RDoc 6.0.1. Due to unknown RDoc issue, RDoc prior to
6.0.0.beta1+ failed to parse the regex in lib/liquid/tag/parser.rb.
Upstream also set "has_rdoc = false" in gemspec.

Use OPTIONS_EXCLUDE=DOCS instead of NOPORTDOCS

Suggested by: mat

Approved by: ports-secteam (blanket)