Allow NPM installs, don't enforce Yarn

[fchristl]

• Removed pre-install hook that enforced Yarn and did not allow NPM
• Updated the ava package as with the old version, the build would not work
• Added @types/npm as without, the build would not work

[fchristl]

This PR addresses https://github.com/juicycleff/casbin-mongodb-adapter/issues/2

[hsluoyz]

lgtm

[hsluoyz]

Hi @juicycleff , can you review it?