Use both minimal and maximal versions in CI

[joshlf]

See also: #1595

By default, when Cargo is performing dependency resolution, it tries to find the maximum version of each dependency which satisfies the dependency constraints. However, this can hide issues such as accidentally depending upon a feature that was introduced after the minimum version specified by a dependency. In order to catch these errors, we should also test with Cargo's minimal-version flag. This flag is unstable, so we'll only be able to do this on the nightly toolchain, and we may have to remove it at some point if the feature breaks or is removed.