Don't overwrite the .env.sample file if it exists

Previously when you run the install ` curl https://raw.github.com/mm53bar/pave/master/install.sh | sh` it overwrote the existing .env.sample file in that directory. 

Since I'm installing pave into an app that already has an .env.sample, I only want the new sample env vars that pave provides, I don't want to blow app-specific .env.sample vars away.