Rspec and bundler setup fixes

Replaces !891 (closed) and !900 (closed).

• We found out that Bundler 1.16.5 fixes a bug that made make rspec fail for some of us. This change updates ruby/Gemfile to require a new enough version of Bundler.
• We also found out that our Rspec integration tests, when invoked as cd ruby && rspec instead of make rspec, would sometimes exercise stale Ruby code. This change also fixes that.
• Include rspec gitaly test logs in the CI artifacts to make it easer to debug CI issues if they arise in the future.