Security vulnerability: Update packages cryptiles and mime

This project depends on vulnerable versions of `cryptiles` and `mime`.

Cryptiles: [CVE-2018-1000620](https://nvd.nist.gov/vuln/detail/CVE-2018-1000620)
Mime: [CVE-2017-16138](https://nvd.nist.gov/vuln/detail/CVE-2017-16138)

It would probably also be a good idea to turn on automated checks for insecure dependencies.

Yarn [resolutions](https://yarnpkg.com/lang/en/docs/selective-version-resolutions/) may be helpful here. For us, pinning `request` to [2.87.0](https://github.com/request/request/pull/2943#issuecomment-390577329) seems to fix this (`request` is the source of the `cryptiles` and `mime` dependencies).