Update poltergeist to version 1.5.1

### Agile Planner Story
[Faster/reliable smart-answers test performance](https://www.agileplannerapp.com/boards/105200/cards/8770)

### Why the upgrade
This appears to fix the JS timeout issues we've been having. 5 consecutive builds have passed on the build server without JS timeouts and the test suite runs on local development machines from start to finish.