Install Needed Prettier/Babel packages

[danielma]

#4 resolves an issue where we weren't installing the right prettier and babel versions.

That PR fixes the immediate issue so projects can move forward, but ideally I think we'd make some changes (maybe to getPeerDependencies) that would check for any reference to prettier or babel and make a point to install all of those packages.

All of this in the interest of installing the fewest packages possible