Fix ZPM urls to use githubusercontent.com and remove trailing slash

This changes ZPM to use the `raw.githubusercontent.com` domain which Github [recently changed](https://developer.github.com/changes/2014-04-25-user-content-security/). It also fixes the double trailing slash for the `package.json` URL (such as `https://...//package.json`).

Before these changes I couldn't install any packages (I was trying the `zedapp/jsx-mode` package).