[minor] no need to download composer via curl

No need to download composer installer via curl and install it by php.
Just to save few secs on Travis jobs
also try to improve on @till 's recommendation [here](http://till.klampaeckel.de/blog/archives/204-Whats-wrong-with-composer-and-your-.travis.yml.html)

```
install: composer install
script: phpunit
```