Fix double directory separators

This was done in a PR that I had to close. Anyway, better on its own branch.

`include_once( HMBKP_PLUGIN_PATH . '/admin/schedule.php' );`

the constant already has the trailing slash, so this causes a double slash.
![slash_ani](https://cloud.githubusercontent.com/assets/30460/3641900/5b415fee-10ba-11e4-95f2-2b5aadfa6dec.gif)
![slash_ani](https://cloud.githubusercontent.com/assets/30460/3641900/5b415fee-10ba-11e4-95f2-2b5aadfa6dec.gif)