Fix the default encryption algo with wrong key size

In `app/config/app.php`,

The doc says the `key` should be `random`, `long string`, but it failed to mention what key size should be. 

The default algo `rijndael-256`'s max key size is `256`, so I suggest to do a `md5` on the `key` inside of `Encrypter`'s constructor. However, leave out the `setKey`, because I believe the callers should know about the algo and key size before setting a different key/algo.

Or should we just update the docs to indicate the default key size should be <= `256 bits`