Update currency.py

Stops the "TypeError: expected string or buffer (source unknown)" error when typing .cur and having no arguments supplied.

Quite the issue in my channel, apparently.
Merge pull request #506 from HarinderToor/patch-1

[currency]Handle the error when no input parameters are specified.
[chanlogs] Don't hardcode home directory

Fixes #496
[youtube] Fix unicode errors
[Many modules] Add unicode_literals import

Also add #coding: utf8 line

This is probably going to force a lot of errors up, but they were
already hiding there anyway.