client version checking

[melange396]

have the python, r, and javascript clients check their version against the current running version on the API server, and print a message informing the user that an update is available that may include fixes or new functionality.

a downside to this is that the version number is incremented MUCH more frequently than changes are made to the client code, so we may often be prompting updates that are pointless.

[melange396]

somewhat related to #1296

[melange396]

We can probably pretty safely ignore this for the R client since it is on the chopping block in #1377 , and ignore the js client as well because it never seemed to really get much (if any!) use.

For the python client, we should make #1431 a prerequisite for this issue.

It would also make sense to do the conditional PyPI release suggested in #1296 and then check the version through pypi.org instead of using our version endpoint:

import requests
requests.get('https://pypi.org/pypi/delphi-epidata/json').json()['info']['version'] # => '4.1.20'