Use default date in gemspec

As suggested by Mr. da Salva. Cleaning up my copy pasta.

Change-Id: I6fd7c5064a3305add5e36bb1227a9b421cade4cf
Reviewed-on: https://gerrit.causes.com/22320
Tested-by: Aiden Scandella <aiden@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>

Author: Aiden Scandella

overcommit.gemspec

@@ -5,7 +5,6 @@ Gem::Specification.new do |s|
   s.name              = 'overcommit'
   s.version           = Overcommit::VERSION
   s.license           = 'MIT'
-  s.date              = '2013-05-15'
 
   s.summary     = 'Opinionated git hook manager'
   s.description = 'Overcommit is a utility to install and extend git hooks'