(#2418) - remove PouchDB Server from allowed failures

Now that [this fix](https://github.com/rvagg/node-levelup/commit/c1c2ae6b5dca143295ba7a847ec25853ff69f6a3) has been npm published to node-levelup, we can safely remove PouchDB Server.  I've been keeping tabs on it in Travis (both Firefox and Node tests), and this seemed to be the last intermittent.