fix(timepicker): Stringify pad return when value >= 10

In some cases, the increment function becomes invalid because the
minutes are invalid. This change ensures that the value returned is a
string

src/timepicker/timepicker.js

@@ -103,7 +103,7 @@ angular.module('ui.bootstrap.timepicker', [])
   }
 
   function pad( value ) {
-    return ( angular.isDefined(value) && value.toString().length < 2 ) ? '0' + value : value;
+    return ( angular.isDefined(value) && value.toString().length < 2 ) ? '0' + value : value.toString();
   }
 
   // Respond on mousewheel spin