Updated browser logo urls to fix missing IE logo

It looks like @alrra now maintains the browser-logos repo, and a [recent commit](https://github.com/alrra/browser-logos/commit/2105b14a1a304b691aeba8b9969e72b69d1aaab5) changed the name of the IE logo.
Updated EN and PT flags to use correct country flags
Added credit to GoSquared for flag icons in README.md