Update core.js

Changed line 1153 to include a lookaround for a digit in the event a minus sign has been matched so that a "-" on it's own does not match.

From
'''JavaScript
return this.optional( element ) || /^-?(?:\d+|\d{1,3}(?:,\d{3})+)?(?:\.\d+)?$/.test( value );
'''

To
'''JavaScript
return this.optional( element ) || /^-?(?=\d)(?:\d+|\d{1,3}(?:,\d{3})+)?(?:\.\d+)?$/.test( value );
'''