Update rustls-pemfile requirement from 1.0 to 2.0

Updates the requirements on [rustls-pemfile](https://github.com/rustls/pemfile) to permit the latest version.
- [Release notes](https://github.com/rustls/pemfile/releases)
- [Commits](rustls/pemfile@v/1.0.0...v/2.0.0)

---
updated-dependencies:
- dependency-name: rustls-pemfile
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

examples/hello_world_tls/Cargo.toml

@@ -7,4 +7,4 @@ edition = "2018"
 
 [dependencies]
 gotham = { path = "../../gotham", features = ["rustls"] }
-rustls-pemfile = "1.0"
+rustls-pemfile = "2.0"