Resolve imports exported using "browser"

This works around rollup/plugins#843.

See the documentation: https://github.com/rollup/plugins/tree/master/packages/node-resolve#exportconditions

rollup.config.addon.js

@@ -31,7 +31,7 @@ export default (cliArgs) => {
           "'https://rally.mozilla.org'",
       }),
       resolve({
-        browser: true,
+        exportConditions: ["browser"],
         // This is required in order for rollup to pick up
         // the correct dependencies for ping encryption.
         preferBuiltins: false,