Add support for Deno

Hi,

I would like to request official support for Deno, as my company is beginning to use it. It would be very useful to have easy access to a preinstalled version of Deno for simplicity and performance reasons.

Deno is very similar to Node.js. In fact, it was created by Ryan Dahl, the creator of Node, for the next generation of JavaScript. So it’s relevant to a huge portion of the Travis CI community.