Pin phantomjs version

Fix a travis npm error

```
npm ERR! peerinvalid The package phantomjs does not satisfy its siblings' peerDependencies requirements!
npm ERR! peerinvalid Peer mocha-phantomjs@3.5.0 wants phantomjs@~1.9.1
```
https://travis-ci.org/ckan/ckan/jobs/34413828#L1068