Switch Travis config to use `arm64` architecture by default

Since Local Server supports Arm builds now we can switch Travis to use Arm chips by default. This should have the following benefits:

- Usually smaller docker image sizes
- Faster execution time

We should start with an experiment and benchmark some existing module tests e.g. for this repo and see what difference using arm makes.

Acceptance criteria:

- [ ] All Altis modules use Arm architecture on travis by default
- [ ] The project level config uses Arm by default
- [ ] The architecture is documented along with how to modify it - https://github.com/humanmade/altis-dev-tools/tree/master/travis
- [ ] The expected speed improvements are documented

## For Altis Team Use

### Ready for Work Checklist

Is this ticket ready to be worked on? See
[the Play Book Definition of Ready](https://playbook.hmn.md/play/product/definition-of-ready/)

- [x] Is the title clear?
- [x] Is the description clear and detailed enough?
- [x] Are acceptance criteria listed?
- [ ] Have any dependencies been identified? (Optional)
- [x] Have any documentation/playbook changes been identified? (Optional)
- [x] Is an estimate or time box assigned?
- [x] Is a priority label assigned?
- [ ] Is this ticket added to a milestone?
- [ ] Is this ticket added to an epic? (Optional)

### Completion Checklist

Is this ticket done? See
[the Play Book Definition of Done](https://playbook.hmn.md/play/product/definition-of-done-2/)

- [ ] Has the acceptance criteria been met?
- [ ] Is the documentation updated (including README)?
- [ ] Do any code/documentation changes meet project standards?
- [ ] Are automatic tests in place to verify the fix or new functionality?
- [ ] Or are manual tests documented (at least on this ticket)?
- [ ] Are any Playbook/Handbook pages updated?
- [ ] Has a new module release (patch/minor) been created/scheduled?
- [ ] Have the appropriate `backport` labels been added to the PR?
- [ ] Is there a roll-out (and roll-back) plan if required?