remove version number from micro:bit firmware URL

This new URL will be a redirect to the actual file, including the
version number. This way we'll be able to deploy firmware updates
without needing to change any content in this repository.

src/views/microbit/microbit.jsx

@@ -105,7 +105,7 @@ class MicroBit extends ExtensionLanding {
                                     <a
                                         download
                                         className="download"
-                                        href="https://downloads.scratch.mit.edu/microbit/scratch-microbit-1.1.0.hex.zip"
+                                        href="https://downloads.scratch.mit.edu/microbit/scratch-microbit.hex.zip"
                                     >
                                         <FormattedMessage id="microbit.downloadHex" />
                                     </a>