Remove 'ignorecase: true' from DateNumbers

This ensures that we don't flag instances of lowercase 'may' -- e.g., "users
may see the following behavior."

Related to #13.

Author: jdkato

Microsoft/DateNumbers.yml

@@ -1,7 +1,6 @@
 extends: existence
 message: Don’t use ordinal numbers for dates.
 link: https://docs.microsoft.com/en-us/style-guide/numbers#numbers-in-dates
-ignorecase: true
 level: error
 nonword: true
 tokens:

fixtures/DateFormat/test.md

@@ -5,3 +5,5 @@ It's 12 Mar 2016. This is okay: 15 and 2017
 It happenened on October twenty-eighth.
 
 For example, 6/12/2017 might be June 12, 2017 or December 6, 2017.
+
+Users may see the following behavior.