trackInputChanges fails silently on older broccoli versions

[asakusuma]

If your app is not using broccoli@^3.2.0, the trackInputChanges option does nothing, meaning that the build() method does not get a list of changes. There is no warning or error shown.

cc @thoov

[rwjblue]

The polyfill (in broccoli-node-info) should be automatically saying every input has changes when the host broccoli version does not support the underlying feature.