Gemfile: refer to default branch "main" in Rails

[olleolleolle]

This PR makes the Gemfile point to the right branch - for Bundler 1.x.

This will repair the test suite for older Ruby versions.

Details

Newer Ruby versions will have issues, in that they are asking for a newer Sprockets, which is not configured with a manifest.js.

The test failures for the newer Ruby versions include cases like:

AssetsTest#test_coffee-script.js_is_included_in_Sprockets_environment:

Sprockets::Railtie::ManifestNeededError: Expected to find a manifest file in `app/assets/config/manifest.js`
But did not, please create this file and use it to link any assets that need to be rendered by your app:

Example:

  //= link_tree ../images
  //= link_directory ../javascripts .js
  //= link_directory ../stylesheets .css

and restart your server

[olleolleolle]

@rafaelfranca I included more context in the PR description, hope this makes it more mergeable.