Fix "Could not find loader 'python3'"

Fixes this error when enabling the plugin under Ubuntu 12.04:

    (gedit:2132): libpeas-WARNING **: Could not find loader 'python3' for plugin 'trailsave'

See http://askubuntu.com/a/302431
Merge pull request #15 from nerab/patch-1

Replace `python3` loader with `python`, closes #16.