Formula equality reflects the active spec
Pass the requested spec into the formula instance
Decouple spec selection from ARGV
Remove ARGV.filter_for_dependencies
libevent: updating homepage
libevent: Changing git add from SF to GH for SSL/TLS

Changing the git address to libevent official Github repo because sourceforge doesn't support SSL/TLS.
libevent: update 2.0.21 bottle.