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.

install.sh

@@ -34,7 +34,7 @@
 
 # Downoad support files
 
-      curl -s -L https://raw.github.com/mm53bar/pave/master/.env.sample -o .env.sample
+      curl -s -L https://raw.github.com/mm53bar/pave/master/.env.sample >> .env.sample
 
 # Set the permissions for the scripts