Support conditional engine loading when Rails is not required (#6)

For people using a very lightweight clock configuration file (without requiring the entirety of Rails), this allows the engine file to not be required while still using the hooks for heartbeats etc.

lib/clockwork_web.rb

@@ -3,7 +3,7 @@
 require "safely/core"
 
 # modules
-require_relative "clockwork_web/engine"
+require_relative "clockwork_web/engine" if defined?(Rails)
 require_relative "clockwork_web/version"
 
 module ClockworkWeb