nosetests -> nosetests3

I believe in Ubuntu 14.04LTS and 16.04LTS, python 2 is still the default environment. In other words, if we type

```python
nosetests
```
It will give you 
```bash
$ nosetests
The program 'nosetests' is currently not installed. You can install it by typing:
sudo apt install python-nose
```
So maybe changing it to

```nosetests3``` on the README.md?