Change the Babel integration recommendation to enforce: 'pre'

[foolip]

See #86 (comment)

[jsf-clabot]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

[foolip]

I can't sign a Individual Contributor License Agreement since I work for Google and this change is not important enough for me to figure out if/how to sign it as a company. Closing.