Do not manually update system RubyGems (mastodon#6355)

Travis CI ships compatible system RubyGems now:
travis-ci/travis-ci#8969 (comment)
> I have repackaged the 2.5.0 archive for Linux to include RubyGems 2.7.4,
> which should have the fix for this issue. Please restart the affected
> jobs, and let us know how they go for you.

.travis.yml

@@ -47,7 +47,6 @@ services:
   - redis-server
 
 install:
-  - gem update --system
   - nvm install
   - bundle install --path=vendor/bundle --without development production --retry=3 --jobs=16
   - yarn install