Build failures installing bundler due to bundler 2.0 release

[nickgsc]

FYI, the explicit call to gem install bundler (introduced in #2) started causing build failures for us this morning as a result of Bundler releasing 2.0, which effectively has a breaking change in needing RubyGems 3.0:

-----> webpack-rails app detected
-----> gem install bundler
ERROR:  Error installing bundler:
	The last version of bundler (>= 0) to support your Ruby & RubyGems was 1.17.3. Try installing it with `gem install bundler -v 1.17.3`
	bundler requires RubyGems version >= 3.0.0. The current RubyGems version is 2.7.6. Try 'gem update --system' to update RubyGems itself.
 !     Push rejected, failed to compile webpack-rails app.
 !     Push failed

I believe a short-term fix for this would be to forcibly set the version to 1.x in the gem install command. I expect this will go away when Heroku formally supports Bundler 2.0/RubyGems 3.0 in the ruby buildpack.