Re-add and apply MDL rule MD040

We were following it about half of the time and we even added it to our
former `.mdlrc` file. However, for some reason, MDL doesn't detect this
rule when specified in the `.mdlrc` file, so we didn't notice we weren't
following it in many cases.

Now that we're using a style file to configure MDL, we can enable this
rule again and apply it, since now MDL correctly includes it in its
report.

README_ES.md

@@ -45,13 +45,13 @@ bin/rake db:dev_seed
 
 Para ejecutar la aplicación en local:
 
-```
+```bash
 bin/rails s
 ```
 
 Para ejecutar los tests:
 
-```
+```bash
 bin/rspec
 ```