refactor: replace deprecated String.prototype.substr()

.substr() is deprecated so we replace it with functions which work similarily but aren't deprecated

Signed-off-by: Tobias Speicher <rootcommander@gmail.com>

src/core/Spec.js

@@ -316,7 +316,7 @@ getJasmineRequireObj().Spec = function(j$) {
       boilerplateEnd =
         boilerplateStart + Spec.pendingSpecExceptionMessage.length;
 
-    return fullMessage.substr(boilerplateEnd);
+    return fullMessage.slice(boilerplateEnd);
   };
 
   Spec.pendingSpecExceptionMessage = '=> marked Pending';