Change Travis configuration to run out of the box

[joshpuetz]

This PR makes several tweaks to allow the CFP app to run on Travis out of the box:

• CFP app now prefers Ruby 2.7.2 according to it's Gemfile, so this PR sets that same Ruby version in travis.yml to avoid errors from Travis about conflicting Ruby versions.
• In addition it adds bundle, nvm, and yarn installation comments to the travis.yml file to make sure Travis test runs have everything available need to run the RSpec test suite.
• Adds an .nvmrc file to specify the required version of node to be installed
• Specifies bundler installs should be cached

[barrettclark]

Thanks for doing this @joshpuetz! One question: we don't typically include the language version dotfiles. The Gemfile handles that now for Ruby. There doesn't seem to be a Node analog in Rails, though. Do you usually commit both the .nvmrc and .ruby-version files in your repos, or just the .nvmrc?

[joshpuetz]

We typically include both as many people on our team use both rvm and nvm to manage multiple rubies and nodes.

That said I've changed the travis.yml file to include the nvm version and not require the .nvmrc file anymore!