Bump compiler

[clbarnes]

Resolves #93

• Updates docs to reflect maturin usage
• Bumps versions of pyo3 and numpy(rust)
• Uses the oldest pyo3-compatible compiler
• Travis tests against release builds

[pattonw]

awesome, thanks for figuring this out Chris. It looks like the pyo3 and numpy are being updated quickly so hopefully we won't be stuck with nightly for too long.

[clbarnes]

The nightly requirement is unfortunately blocked by a rust feature with no timeline for stabilisation (I linked the tracking issue in #93).

[pattonw]

This all looks pretty good to me, can I go ahead and merge it?

[clbarnes]

Yep! I don't think there's anything else to change in the docs right now. I think the deploy stage still isn't running, though.

[pattonw]

Yeah, the deploy stage still isn't running, but I can try to sort that out later