Pin Bundler to 1.17.3 to avoid using 2.x
This MR pins Bundler to 1.17.3 to avoid using Bundler 2.x/RubyGems 3.x.
cf. https://bundler.io/blog/2019/01/04/an-update-on-the-bundler-2-release.html
Blocks !569 (merged) !570 (closed) !574 (merged)
Replaces !572 (closed)
Closes #437 (closed)