repl: fix persistent history and env variable name

Issue #1575 did introduce a check for options.terminal but this variable
wasn't able to get truthy, which in turn broke persistent history
completely. This changes the variable to get truthy on true terminals.

Additionally, the docs and the code did differ on which environment
variable was used for history. This changes the code to use
NODE_REPL_HISTORY_FILE.

PR-URL: https://github.com/iojs/io.js/pull/1593


Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>