Added missing semicolon in supervisor.js

I'd propose either adding the semicolon here to the end of line 12, or adding a comment before line 12 which explains the purpose of omitting the semicolon.  If there is a reason, feel free to comment and I'll modify my pull request appropriately.

Credit to https://github.com/isaacs/node-supervisor/pull/129/ for pointing out the missing semicolon.  I just thought this commit would simplify the review process.