Remove trailing slash from perference namespace

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.
Merge pull request #1 from darraghb/feature/fix-preference-creation

Remove trailing slash from perference namespace