Add RealDigits (courtesy of Larion)

[rocky]

From #691

In this pull request, the RealDigits[] function has been added to the Mathics.
In order to pass all the TravisCI checks, I've also updated the .travis.yml file by adding the package Requests and specifying the version of Ipython to 5.0.0.

Note: TravisCI update occurred previously for other reasons