Update travis configuration
Update travis configuration

- Improve updating nvm
- Remove external shell calls.

rolandgeider/wger,petervanderdoes/wger,rolandgeider/wger,wger-project/wger,kjagoo/wger_stark,wger-project/wger,kjagoo/wger_stark,kjagoo/wger_stark,petervanderdoes/wger,petervanderdoes/wger,wger-project/wger,kjagoo/wger_stark,rolandgeider/wger,wger-project/wger,petervanderdoes/wger,rolandgeider/wger