Bump stamp to ~> 0.6.0

Closes #2801

Gemfile

@@ -63,7 +63,7 @@ gem 'rack-cors',    '~> 0.4.0', require: 'rack/cors'
 # Format dates and times
 # based on human-friendly examples
-gem "stamp", '~> 0.5.0'
+gem "stamp", '~> 0.6.0'
 # Enumeration fields
 gem 'enumerize', '~> 0.7.0'

Gemfile.lock

@@ -689,7 +689,7 @@ GEM
       actionpack (>= 3.0)
       activesupport (>= 3.0)
       sprockets (>= 2.8, < 4.0)
-    stamp (0.5.0)
+    stamp (0.6.0)
     state_machine (1.2.0)
     stringex (2.5.2)
     systemu (2.6.5)
@@ -909,7 +909,7 @@ DEPENDENCIES
   spring-commands-spinach (~> 1.0.0)
   spring-commands-teaspoon (~> 0.0.2)
   sprockets (~> 2.12.3)
-  stamp (~> 0.5.0)
+  stamp (~> 0.6.0)
   state_machine (~> 1.2.0)
   task_list (~> 1.0.2)
   teaspoon (~> 1.0.0)