Rewrite to use docker-images

[vweevers]

Should we deprecate this in favor of https://github.com/prebuild/docker-images?

[vweevers]

On second thought, we can repurpose this.

With the exception of Windows & Mac, leveldown's prebuilds are built in docker:

https://github.com/Level/leveldown/blob/f637eb79d52b211494e61e47610a5c6112f4f25a/package.json#L20-L26

Currently spread out over 3 travis jobs. We could make that one job, that calls prebuildify-cross, which will do all of the above (i.e. building for all platforms by default). Then prebuildify-cross can be reused by other projects. Ideally that'd be as simple as adding one dependency, so we may want to copy some behaviors from prebuild-ci (checking whether it needs to run and doing uploads, which in the prebuildify* setup is handled by travis).