Remove trailing slash from perference namespace

Hit a problem where the plugin preferences were not initialising for me.

Guessing that you didn't encounter this problem because you already had the preferences created locally.

Commit Message of fix:
"Pidgin fails to create initial preferences when the namespace
for multiple preferences is created with a trailing slash.

When calling:
    Purple::Prefs::add_none($PLUGIN_PREFS);

PLUGIN_PREFS must not end with a '/' otherwise it will prevent all
settings placed under this namespace from being created on first
load of the plugin."