`README.md` and main executable contradict each other with respecto to python version.

Shebang line of [`arandr-indicator.py`](https://github.com/denilsonsa/arandr-indicator/blob/master/arandr-indicator.py) requires python 3:

```
#!/usr/bin/env python3
```

But [`README.md`](https://github.com/denilsonsa/arandr-indicator/blob/master/README.md) states that python 2.7 is required:

```
Requirements
[...]
* Python 2.7
```

Regards!
