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

@@ -24,7 +24,6 @@ def call
 
   class Commands::SlowCommand < Commands::BaseCommand
     def call
-      Timecop.travel Time.zone.now + 1
       :foo
     end
   end
@@ -52,9 +51,15 @@ 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(1472243573027000, 1472246723347000)
+
       expect(PublishingAPI.service(:statsd)).to receive(:timing) do |name, time, sample_rate|
         expect(name).to eq "Commands.SlowCommand"
-        expect(time).to be_within(10).of(1000)
+        expect(time).to eq(3150)
         expect(sample_rate).to eq 1
       end