Update rollup to latest version, 2.50.5.

It appears our closing of this old @renovate PR to update Rollup to v2
caused @renovate to stop opening PRs to update Rollup to any v2.x.y
version, so our version of Rollup has been out of date since then:
#6033

Though obviously not ideal, this major version lag hasn't been a problem
for us because Rollup v1 still works very well, and we use it only to
generate CommonJS bundles, which is a relatively stable build target
(CommonJS hasn't changed much lately). We rely on other tools like tsc
for critical stuff like TypeScript compilation, and we have numerous
ways to verify the output of our build system.

With that said, Rollup v2 has some great new features that I'm looking
forward to trying, such as `output.inlineDynamicImports`.

package.json

@@ -116,7 +116,7 @@
     "recompose": "0.30.0",
     "resolve": "1.20.0",
     "rimraf": "3.0.2",
-    "rollup": "1.31.1",
+    "rollup": "2.50.5",
     "rollup-plugin-terser": "7.0.2",
     "rxjs": "6.6.7",
     "subscriptions-transport-ws": "0.9.18",