Broken semver promise with 0.4.5

[nyurik]

Thx for maintaining this! I just upgraded from 0.4.4 to 0.4.5 and it caused a lot of compilation failures in the martin project:

error[E0423]: expected value, found struct `Etag`
  --> martin/src/srv/fonts.rs:23:12
   |
23 |     wrap = "Etag",
   |            ^^^^^^
   |
  ::: /home/nyurik/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/actix-middleware-etag-0.4.5/src/lib.rs:55:1
   |
55 | pub struct Etag {
   | --------------- `Etag` defined here

Should there be a semver checker step as part of the CI?