Travis CI: simplify config

[dhardy]

Closes #867

[BurntSushi]

Thanks! I think needing to add direct dependencies in order to make minimal versions work only happens because those dependencies themselves don't have minimal versions specified correctly? Or was there some other issue? I'd be happy to help submit PRs to those dependencies to make minimal versions work more smoothly.

[dhardy]

I'm actually not sure. @vks is the author of average which depends on conv but it may be that the issue we hit with the min version is not actually a bug in average per se.

The other issue is with serde_derive which may be a "bug" @dtolnay: serde depends on serde_derive version 1.0, thus us depending on serde >= 1.0.63 does nothing.

[newpavlov]

Looks good! I think it would be also nice to add 32-bit tests, e.g. you can kill two birds by adding a WASM job.

[newpavlov]

You can now write it as:

name: "Linux, 1.32.0"
env: ALLOC=0

[newpavlov]

Do we really need macOS and iOS tests?

[dhardy]

I don't know; possibly not. These aren't too slow in any case (unlike the WASM tests).

[vks]

Looks good to me! I'm not sure what the issue with the minimal conv version is, but most likely average should bump the minimal version (and test it) as @BurntSushi suggested. However, I think this can be done later.