master branch might not exist

After a fresh install I was unable to run the installer again. It was breaking with the error

error: branch 'master' not found.

my fix makes sure that the installer continues after that.
Should this not be:
```sh
cd $NVM_DIR && git checkout v0.11.2 && (git branch -D master || true)
```
Otherwise if any of the before mentioned commands fails it always end with success, right?