Replace Timecop with Process.clock_gettime in spec

This test mocks the internals of Statsd (time sending a message to timing)

Statsd no longer uses Time.now to determine the time, instead
using Process.clock_gettime in order to compute monotonic time.
More details: reinh/statsd#70.

We therefore must update our test to mock the new call and also
stub the constant Process::CLOCK_MONOTONIC.

This is the same dependency on some Statsd internal behaviour, it
just _looks_ more brittle.
Timecop may support this in future: travisjeffery/timecop#220 (comment)

spec/commands/base_command_spec.rb

@@ -72,6 +72,12 @@ def call
 
   describe "timing" do
     it "sends a command's duration to statsd" do
+      stub_const("Process::CLOCK_MONOTONIC", 6)
+      allow(Process).to receive(:clock_gettime)
+      allow(Process).to receive(:clock_gettime)
+        .with(Process::CLOCK_MONOTONIC, :nanosecond)
+        .and_return(1_472_243_573_027_000, 1_472_246_723_347_000)
+
       expect(PublishingAPI.service(:statsd)).to receive(:timing) do |name, time, sample_rate|
         expect(name).to eq "Commands.SlowCommand"
         expect(time).to within(100).of(1000)