bump rails from 6.0.3.2 to 6.0.3.4
bump rails from 6.0.3.2 to 6.0.3.4

This is a very small bump, incorporating the two most recent security
releases of rails 6.0. None of the fixed security issues look like
they'd impact this app, so updating is mainly about quietening
dependabot.

buildkite/docs,buildkite/docs,buildkite/docs,buildkite/docs