Minimum rollup version isn't high enough for @rollup/plugin-node-resolve

In our project we updated to latest version of workbox, and as a result we get this error:

```
ERROR in Insufficient rollup version: "@rollup/plugin-node-resolve" requires at least rollup@2.78.0 but found rollup@2.77.0.
```

Indeed, the current dependency for `@rollup/plugin-node-resolve` in `workbox-build` is 15.2.3 (see [package.json](https://github.com/GoogleChrome/workbox/blob/f4a49dac92d043926505c83a02ebeea5c5bf1957/packages/workbox-build/package.json#L28)), which clearly requests rollup 2.78+ in its [readme file](https://github.com/rollup/plugins/blob/master/packages/node-resolve/README.md).
However the minimum rollup version in workbox-build [is 2.43.1](https://github.com/GoogleChrome/workbox/blob/f4a49dac92d043926505c83a02ebeea5c5bf1957/packages/workbox-build/package.json#L39), therefore when a project updates the workbox dependency, rollup itself stays at the older version and isn't updated.

The solution should be fairly easy: rollup needs to be updated to a more recent version. Hopefully it still works? (but I guess you'd know from other users).