Use rvm-prompt options instead of sed

`rvm-prompt` supports parameters to change the printed string. This PR uses this feature in place of the sed expression.

```
$ rvm use 2.1.2@myproject
$ rvm-prompt u g
➋➀@myproject
$ rvm-prompt v g
2.1.2@myproject
$ rvm-prompt i v g
ruby-2.1.2@myproject
$ rvm-prompt   # default
ruby-2.1.2@myproject
```