remove Cargo.lock from .gitignore

To ensure reproducible builds for the compiled binaries it's recommended
to add the `Cargo.lock` file to version control.

Refs.:
- Cargo Book FAQs: https://doc.rust-lang.org/cargo/faq.html#why-do-binaries-have-cargolock-in-version-control-but-not-libraries
- Discussion in a github issue: rust-lang/cargo#315

.gitignore

@@ -1,2 +1 @@
 target/
-Cargo.lock