Replace deprecated String.prototype.substr() (#623)

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/stringify.js

@@ -7,7 +7,7 @@ import validate from './validate.js';
 const byteToHex = [];
 
 for (let i = 0; i < 256; ++i) {
-  byteToHex.push((i + 0x100).toString(16).substr(1));
+  byteToHex.push((i + 0x100).toString(16).slice(1));
 }
 
 export function unsafeStringify(arr, offset = 0) {

src/version.js

@@ -5,7 +5,7 @@ function version(uuid) {
     throw TypeError('Invalid UUID');
   }
 
-  return parseInt(uuid.substr(14, 1), 16);
+  return parseInt(uuid.slice(14, 15), 16);
 }
 
 export default version;