make require 'timecop' a bit more threadsafe

Every few hundred runs of our application we are seeing something like:
```
22:27:36 NameError: uninitialized constant #<Class:Time>::Timecop
22:27:36 /build/vendor/bundle/ruby/2.4.0/gems/timecop-0.9.1/lib/timecop/time_extensions.rb:7:in `mock_time'
22:27:36 /build/vendor/bundle/ruby/2.4.0/gems/timecop-0.9.1/lib/timecop/time_extensions.rb:14:in `now_with_mock_time'
...
```

I think this is because we are running multiple ruby `Thread`s, and one thread is doing `require 'timecop'` and is interrupted after monkeypatching `Time` but before creating the `TimeCop` constant. We should do the monkeypatching after the constant is available.

lib/timecop/timecop.rb

@@ -1,5 +1,4 @@
 require 'singleton'
-require File.join(File.dirname(__FILE__), "time_extensions")
 require File.join(File.dirname(__FILE__), "time_stack_item")
 
 # Timecop
@@ -235,3 +234,6 @@ def initialize
     end
   end
 end
+
+# This must be done after TimeCop is available
+require File.join(File.dirname(__FILE__), "time_extensions")