npm install on fresh clone overwrites enclave.js

[levithomason]

When cloning a project that uses enclave and running npm i, enclave runs all of it's prompts again and hoses the existing enclave.js.

I think it is running it's ropes post install but not checking if the project is already configured with enclave. If the project already has an enclave.js, enclave should probably not do anything on install.