replace deprecated String.prototype.substr() (#876)

String.prototype.substr() is deprecated (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr) so we replace it with slice() which works similarily but isn't deprecated.
Signed-off-by: Tobias Speicher <rootcommander@gmail.com>

src/common.js

@@ -179,7 +179,7 @@ function setup(env) {
 			namespaces = split[i].replace(/\*/g, '.*?');
 
 			if (namespaces[0] === '-') {
-				createDebug.skips.push(new RegExp('^' + namespaces.substr(1) + '$'));
+				createDebug.skips.push(new RegExp('^' + namespaces.slice(1) + '$'));
 			} else {
 				createDebug.names.push(new RegExp('^' + namespaces + '$'));
 			}