Clean up Makefile

• Use DISTVERSIONPREFIX instead of DISTNAME
• Use = instead of += for PLIST_SUB
• Convert to options target helper
• Use bsd.port.mk instead of bsd.port.pre.mk + bsd.port.post.mk
• Sort PLIST
• Update pkg-descr and remove sentences of outdated Ruby versions [1]
• Update WWW

Reference: https://github.com/jruby/jruby/wiki/AboutJRuby [1]