Update axios dependency to 0.21.1

[davidfishlock]

Addresses a high severity security advisory on axios peer dependency:
https://www.npmjs.com/advisories/1594

[mpyw]

Build seems to be failing 😢
Duplicates of #33 (But I don't close any PR until one of them is merged)

[davidfishlock]

@mpyw the build failures are unrelated to this PR. All builds on this repository will have been failing since 20th October, when node 15 became the stable version.

The Travis config runs against two node versions:

• stable: currently v15.5.1 (version would have changed to 15 on 20th October)
• lts/*: currently v14.15.4

The build is failing on node 15 because the test run is generating unhandled promise rejections. You can see these in the log output for build runs of the last successful PR on this repo:

(node:6693) UnhandledPromiseRejectionWarning: Error: EBADF: bad file descriptor, close

The difference in node 15 is that these unhandled promise rejections will terminate the process. So currently any build on this repo will be passing on node < 15 and failing on node 15+.

I can't reproduce these file descriptor failures locally, this could be something specific to the Travis or linux environment and/or something specific to the way the tests are being set up.

These will either need to be fixed for any future PRs to pass on this repo, or the Travis runs will need to stop running on node 15.

[mpyw]

Fix CI builds · Issue #35 · mpyw/axios-case-converter