Make more robust install-script

I substituted the check for sudo in both scripts because i was getting:

```
$ sudo ./install.sh
./install.sh: 3: [: Illegal number:
```
Instead I refactored to use `set -eu` which should do the trick. Just let the user know what happens. I.e. this is more descriptive:
```
cp: cannot create regular file ‘/usr/bin/pirate-get’: Permission denied
```
Also I refactored to user `$PY_VERSION`, it looks more clean. I don't know with regards to compatibility.