Because the gem name is "amplitude-rb" instead of "amplitude", this will
cause Bundler to `require 'amplitude-rb'` but there's no such file,
which is why the README originally showed the inclusion of `require:
'amplitude'` at the end of the Gemfile line. However, it's more
user-friendly to simply include a placeholder file with the same name as
the gem that loads up the appropriate other file.