build for each version of package submitted to crates.io in two variants for ubuntu/windows
The builds depend on the compiler version and feature flags and maybe even environment (available system libraries). This is not as trivial as building “two” versions.
A “global” cache of build artifacts, indexed by hash, might be interesting (and quite trendy – feel free to add “block chain” and “distributed” to the description). I am however not sure if it’s truly faster than just using cache: cargo in your Travis config (which caches ~/.cargo and ./target on some storage device at Travis’).