Update for new nengo-bones release

[drasmuss]

Motivation and context:

https://github.com/nengo/nengo-bones/pull/14 removes conda from the TravisCI setup, so everything is done with pip instead. This simplifies and speeds up the build process. This PR updates .nengobones.yml to be consistent with that new approach.

Interactions with other PRs:

Depends on https://github.com/nengo/nengo-bones/pull/14

How has this been tested?

All TravisCI checks are passing.

How long should this take to review?

• Quick (less than 40 lines changed or changes are straightforward)

Types of changes:

• Non-code change (touches things like tests, documentation, build scripts)

Checklist:

• [x] I have read the CONTRIBUTING.rst document.
• [x] I have updated the documentation accordingly.
• [ ] I have included a changelog entry.
• [x] I have added tests to cover my changes.
• [x] I have run the test suite locally and all tests passed.

[hunse]

This needs to be rebased onto #216 for tests to pass.