Enable sync on logfiles after calling reopen

Calling `.reopen` on `STDOUT` and `STDERR` disables `sync`, buffering the log files in memory. This can lead to losing the log errors if the process dies unexpectedly.

````
2.1.1 :001 > STDERR.sync
 => true 
2.1.1 :002 > STDERR.reopen 'err.log', 'a'
 => #<IO:err.log> 
2.1.1 :003 > STDERR.sync
 => false 
````

This PR enables `sync` again after calling reopen