Explicitly add node_modules to cacheDirectories for Heroku

[rogermparent]

Adding node_modules to cacheDirectories seems to download a more complete cache than the default behavior, despite the node buildpack docs saying node_modules are cached by default.

While out of the scope of a git PR, another part of reducing rebuild time on Heroku is setting the environment variable YARN_PRODUCTION to false so Heroku doesn't waste over a minute pruning devDependencies.
I tried just moving the Gatsby packages from devDependencies to dependencies, but it takes nearly as long to prune a minimal set of devDependencies as it does with all of them.
Even without pruning devDependencies we're way below the slug max, and I don't see it growing a few hundred MB any time soon. This will be even less of a problem once images are removed from the source repo, though if the images do pose a problem we should be able to remove them like public and .cache after the build.

[rogermparent]

Hold on, I'll re-do this from a branch within the repo so we don't have to authorize the deploys.