Ensure Python3 before reading README with encoding arg

[nokome]

If you accidentally try to install using Python 2 you get this error:

    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/home/nokome/nokome/repo2docker/setup.py", line 5, in <module>
        with open('README.md', encoding="utf8") as f:
    TypeError: 'encoding' is an invalid keyword argument for this function

Ideally, the user would be asked to use Python 3. If you remove the encoding argument, and try again then pip gives us this, more informative message:

$ python -m pip install -e .
Obtaining file:///home/nokome/nokome/repo2docker
jupyter-repo2docker requires Python '>=3.4' but the running Python is 2.7.12

This little PR implements a similar behaviour, whilst also reading the README using the correct encoding.

[betatim]

Seems like a good thing to do.

One pondering: should we make the reading of the README conditional on the Python version? For the case were we are on Python 2 we can set it to an empty string and then let pip generate the error message. The advantage would be that it shows the actual minimum required (currently 3.4 but maybe in the future we start requiring something newer and then "Python 3" isn't good enough anymore).

[nokome]

One pondering: should we make the reading of the README conditional on the Python version?

Yes, that's a better approach! I'll modify.

[nokome]

Now, when trying to install with Python 2 gives the error:

> python -m pip install -e .
Obtaining file:///home/nokome/nokome/repo2docker
jupyter-repo2docker requires Python '>=3.4' but the running Python is 2.7.12

[betatim]

Merge away once the robots are happy.