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

Previously when you run install it overwrites the file that's already there. If I install this into an app directory that already has a .env.sample I lose all the sample env vars for that app.

Similar to https://github.com/mm53bar/pave/commit/31a0ef600c5b891e5efddb87aed0568d89c94a58
Merge pull request #17 from jbabe/dont-clobber-existing-env-sample

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