Replace Timecop with 1 second sleep

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.

I initially updated our test to mock the new call and also
stub the constant Process::CLOCK_MONOTONIC.
But, this is the same dependency on some Statsd internal behaviour, it
just _looked_ more brittle.
Timecop may support this in future: travisjeffery/timecop#220 (comment)

Instead, sleeping for 1 second doesn't also test rely on testing the
internals of the Statsd gem.

spec/commands/base_command_spec.rb

@@ -24,7 +24,7 @@ def call
 
   class Commands::SlowCommand < Commands::BaseCommand
     def call
-      Timecop.travel Time.zone.now + 1
+      sleep 1
       :foo
     end
   end
@@ -54,7 +54,7 @@ def call
     it "sends a command's duration to statsd" do
       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 within(100).of(1000)
         expect(sample_rate).to eq 1
       end