Fixes an oversight in the build-system versioning.

Upstream-Bug: https://github.com/HowardHinnant/date/issues/583

--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -17,7 +17,7 @@
 
 cmake_minimum_required( VERSION 3.7 )
 
-project( date VERSION 2.4.1 )
+project( date VERSION 3.0.0 )
 
 include( GNUInstallDirs )