Default to port 3000 while allowing override (#124)

With this configuration `bin/dev` will use port 3000,
but `bin/dev -p 3001` will correctly start the server on port 3001.

Author: elia

lib/install/Procfile.dev

@@ -1,2 +1,2 @@
-web: unset PORT && env RUBY_DEBUG_OPEN=true bin/rails server
+web: env RUBY_DEBUG_OPEN=true bin/rails server
 css: yarn build:css --watch

lib/install/dev

@@ -5,4 +5,7 @@ if ! gem list foreman -i --silent; then
   gem install foreman
 fi
 
+# Default to port 3000 if not specified
+export PORT="${PORT:-3000}"
+
 exec foreman start -f Procfile.dev "$@"