don't pass / to `imageNamed:`

This is responsible for the majority of crashes on Yosemite. Hope this works. I’m going to have a couple of users try it.

The fix is based on [a clue in the release notes][1]. I made a small project that threw a bunch of strings at `[NSImage imageNamed:]` and including a `/` definitely throws the exception. Couldn’t find any other problematic characters.

[1]: http://news.softpedia.com/news/Apple-Releases-Xcode-6-1-1-GM-with-Updates-to-Swift-App-Extensions-465033.shtml