Add `gem update --system` to before install on travis
Add `gem update --system` to before install on travis

Applies a workaround for bundler 1.16.1 and rubygems 2.7.3
version incompatability by updating to a patched version
of rubygems prior to running rake tasks.

Fixes `bundler: failed to load command: rake`

see https://github.com/travis-ci/travis-ci/issues/8978

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