Use rspec-retry to get past random phantomjs failures

There are random rspec failures in backend's feature tests. This is caused by a well known issue in phantomjs. Sometimes it doesn't load the page element on time before the test start checking on it. There are various workarounds to deal with it: increase wait time, add sleep in failing tests, retry failling tests, etc.

This PR leverages rspec-retry gem to retry failling tests. It's limited only to features tests in frontend & backend where phantomjs is used. It's turned off by default. If anybody wants to retry failling tests they can run rspec like this:
```
RSPEC_RETRY_COUNT=2 rspec
```