add junit formatter for rspec so Jenkins can publish results

## motivation

These Jenkins jobs:

* http://ljs-jenkins.ngrok.com/job/WeTap-API/
* http://ljs-jenkins.ngrok.com/job/WeTap-API-Pull-Requests/

want jnuit style reports.

```
# run rspec normally
be rake spec
# to generate reports
$ be rake spec GENERATE_REPORTS=true
```
## needs review

- [ ] rolled back rspec-rails to '~> 2.14.2' from '3*' because there are no rspec junit formatters for 3.*