Bitbucket Cloud apps that load the JS API break in IE11

From: https://ecosystem.atlassian.net/browse/DEVHELP-2330

1. Any app for Bitbucket Cloud that wants to use the JS API will load https://bitbucket.org/atlassian-connect/all.js
2. This script will in turn load https://d301sr5gafysq2.cloudfront.net/9b9e91d45dab/dist/connect/v5/dist/connect/plugin.min.js
3. This minified script has arrow functions (=>) in it, which are a SyntaxError in IE11. This breaks any app page that relies on the JS API in IE11.
4. There is no official documentation of support being dropped for IE11.

The fix for this should be having whatever minifier you're using minify to ECMAScript 5.