correct TypeError with string formatting

Small tweak to correct this error I found.

```
$ supernova-keyring -s global user

[Keyring operation] Preparing to set a password in the keyring for:

  - Environment  : global
  - Parameter    : user

If this is correct, enter the corresponding credential to store in your keyring
or press CTRL-D to abort:

Traceback (most recent call last):
  File "/home/carl/.xdg/data/virtualenv/supernova-git/bin/supernova-keyring", line 9, in <module>
    load_entry_point('supernova==0.9.9', 'console_scripts', 'supernova-keyring')()
  File "/home/carl/.xdg/data/virtualenv/supernova-git/lib/python2.7/site-packages/supernova/executable.py", line 107, in run_supernova_keyring
    credentials.set_user_password(args)
  File "/home/carl/.xdg/data/virtualenv/supernova-git/lib/python2.7/site-packages/supernova/credentials.py", line 157, in set_user_password
    "supernova service.\n" % (colors.gwrap("Success"), username))
TypeError: not all arguments converted during string formatting
```