Fixes for Bundler 2.0 and Rails's ruby-version syntax

[dazuma]

Several fairly important fixes that have been pending for a while:

• If a user locks their bundle using the recently released bundler 2.0, their app will fail to build because the builder installs bundler 1.16. Fixed by installing both bundler 1 and bundler 2.
• Rails 5.2 generates a non-standard format for the .ruby-version file that the builder didn't know how to interpret. Fixed by mapping Rails's form to the canonical form.

Also:

• Added the bundler install archives to the ruby-build-tools image, and updated the generated Dockerfile to install from there rather than over the internet. This should be a small improvement to build latency and stability.
• Set a smaller MALLOC_ARENA_MAX according to current best practices for optimizing glibc-malloc for Ruby.
• Updated embedded NodeJS to version 10.15.1
• Updated embedded gcloud sdk to version 232.0.0