[Cashtab] Upgrade @bitgo/utxo-lib

Summary: Need this to use latest version of node. The breaking change from 8 to 9 is related to PSBT functions that are not used by Cashtab. See https://github.com/BitGo/BitGoJS/blob/master/modules/utxo-lib/CHANGELOG.md

Test Plan: npm test, npm run build

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15376