Memory usage stats plus weight reduction

Added:
- stricter version requirements on some gems in Gemfile - tests do not run when using newest versions of gem dependencies.
- a way to disable timeseries collection (they generate a lot of noise in redis keys and do not have high value in all use cases).
- memory usage stats collection when NewRelic gem is present (it contains a very useful abstraction helper to get accurate process memory usage on variety of platforms).

Removed:
- requirement for monitored job classes to be loadable in resque-web sinatra app; for most use cases job classes contain plenty of references to rails and app classes overall; loading all of this extra weight is wasteful and unnecessary.