update version of prop-types to >=15.6.2

[Bullfrog1234]

There is an alert on auditing in NPM and YARN that raises this package, a trace of the issue shows that it is in node-fetch

The link is this:
react-ionicons > prop-types > fbjs > isomorphic-fetch > node-fetch

prop-types removed dependency on fbjs in version 15.6.2 by inlining some helpers from it. Current version in package is 15.5.10

Suggest update the version of prop-types to >=15.6.2.

This upgrade may have breaking change, I have not assessed.

Link to prop-types pull request for context.