Increase minimum supported rustc version to 1.28.0.

[iliekturtles]

Changed in anticipation of adding #[repr(transparent)] in #145.
Zero-cost codegen note regarding rustc 1.25.0 is now no longer
relevant.

[iliekturtles]

Build failing for 1.28.0 with the following error:

error: failed to parse manifest at `D:\Code\Test\rust\uom\uom-macros\Cargo.toml`

Caused by:
  editions are unstable

Caused by:
  feature `edition` is required

this Cargo does not support nightly features, but if you
switch to nightly channel you can add
`cargo-features = ["edition"]` to enable this feature

edition keyword is now recognized by cargo but not yet stabilized. Previously builds for 1.24.1 would still pass because the edition keyword was not recognized and hence ignored by cargo. May need to do a 1.28.0 / 1.31.0 split.