Update for Nim 1.3.1

Change `startOfDay` and `trimNanoSec` to use `initDateTime` instead of
mutating the new copy via the deprecated property accessors.

Updated the test suite to reflect functions that moved from this library
to the standard library. We're still testing the expected functionality
to make sure that the contract is maintained for users of this library.

timeutils.nimble

@@ -1,10 +1,10 @@
 # Package
 
-version       = "0.5.2"
+version       = "0.5.3"
 author        = "Jonathan Bernard"
 description   = "Utility methods to fill in the lacking time support in Nim\'s stdlib."
 license       = "BSD3"
 
 # Dependencies
 
-requires "nim >= 1.0.0"
+requires "nim >= 1.3.1"