Changed how MF_URI is set

Fixes an issue on WordPress installations that require SSL on admin pages.

Newer versions of Chrome default to blocking assets loaded from insecure sources, 
and the current method for determining the MF_URI constant doesn't take SSL into account.
added missing paren. ooops.

Edited the file online in github, and totally missed adding the additional paren.
Merge pull request #170 from C3-Presents/master

Fix for loading assets on admin pages when SSL is required