Fix check when no command is supplied

Closes #666.

Author: mernen

lib/spring/client/rails.rb

@@ -20,7 +20,7 @@ def call
 
         if COMMANDS.include?(command_name)
           Run.call(["rails_#{command_name}", *args.drop(2)])
-        elsif command_name.start_with?("db:")
+        elsif command_name&.start_with?("db:")
           Run.call(["rake", *args.drop(1)])
         else
           require "spring/configuration"