Use HTTPS URL for Rails git

Instead of Git protocol which Bundler uses when specifying the "github"
option.
Fix deprecation warning about t.timestamps with no args

For reference, the deprecation warning:

> `#timestamp` was called without specifying an option for `null`. In
> Rails 5, this behavior will change to `null: false`. You should
> manually specify `null: true` to prevent the behavior of your existing
> migrations from changing.