Fallback to Time.now for rubies that don't support Process.clock_gettime

Follow-up on https://github.com/Shopify/statsd-instrument/pull/42.

`Process.clock_gettime` was only added to Ruby [in version `2.1.0`](https://github.com/ruby/ruby/blob/v2_1_0/NEWS#L113-L114). Older versions do not have a way to access the monotonic clock so we should fallback to `Time.now`.

Ping @wvanbergen