repl: fix for a+ fd clearing the file on read

The second step of augmenting the internal REPL with persistent
history was to re-open the history file with a 'w' handle. This
truncated the file. If a user did not enter a new line before
closing the REPL, their history would be deleted.

PR-URL: https://github.com/iojs/io.js/pull/1605


Reviewed-By: Roman Reiss <me@silverwind.io>