cargo: prep for 3.0.0

Our update to chrono 0.4 changes one of our existing public interfaces
and therefore requires a major version bump.

Cargo.toml

 [package]
 name = "git-workarea"
-version = "2.0.2"
+version = "3.0.0"
 authors = ["Ben Boeckel <ben.boeckel@kitware.com>"]
 license = "MIT/Apache-2.0"
 description = """
@@ -8,7 +8,7 @@ Simple routines to work with git repositories and set up minimal workareas with
 them.
 """
 repository = "https://gitlab.kitware.com/utils/rust-git-workarea"
-documentation = "https://docs.rs/git-workarea/~2.0"
+documentation = "https://docs.rs/git-workarea/~3.0"
 readme = "README.md"
 keywords = ["git", "utility"]