Update excon gem to v0.89.0

excon v0.71.1 ships its own cacert.pem dated 2019, which could cause SSL
verification failures for certain services under certain conditions.

Update this gem to the latest version to get a more recent copy and the
latest bug fixes.

Diff: https://my.diffend.io/gems/excon/0.71.1/0.89.0

Changelog: fixed