Add missing dep on OpenSSL

Fixing that error

irb(main):006:0> pd = PagerDuty::Full.new(apikey = "XXXX", subdomain = "CORP")
=> #<PagerDuty::Full:0x007faa1c1afe50 @apikey="XXXX", @subdomain="CORP">
irb(main):007:0> schedule = pd.Schedule.search()
NameError: uninitialized constant PagerDuty::Full::OpenSSL
	from /Users/oaouf/.rbenv/versions/2.0.0-p451/lib/ruby/gems/2.0.0/gems/pagerduty-full-0.1.2/lib/pagerduty/base.rb:20:in `api_call'
	from /Users/oaouf/.rbenv/versions/2.0.0-p451/lib/ruby/gems/2.0.0/gems/pagerduty-full-0.1.2/lib/pagerduty/resource/schedule.rb:20:in `search'
	from (irb):7
	from /Users/oaouf/.rbenv/versions/2.0.0-p451/bin/irb:12:in `<main>'