Pin bundler version to 1.17.2
With Ruby 2.6.3, Bundler is bundled (sic). It uses 1.17.2, but we had 1.17.3 defined in Gemfile.lock (probably before Ruby 2.6.3 was introduced). This change pins Bundler to 1.17.2, so that it doesn't throw a warning every time we run 'bundle update'.
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