IE needs polyfills

[microbit-carlos]

Current master will complain about:

SCRIPT5009: 'Symbol' is undefined
intel-hex.browser.js (165,5)

This is because the intel-hex library has been trasnpiled with babel, but still needs polyfills.

Testing the addition of https://cdnjs.com/libraries/babel-polyfill as a script tag fixes this issue.

[microbit-carlos]

PR: #117

[microbit-carlos]

We also need to add IE polyfills to the microbit-fs library.

[microbit-carlos]

The polyfills for the primary editor are already deployed, and the polyfills for the microbit-fs library are already included in the version of the library deployed in https://python.microbit.org/v/beta.