Ignore Cargo.lock

In Cargo Guide[1] anf Cargo FAQ[2], it is recommended to ignore
`Cargo.lock`.

[1]: http://doc.crates.io/guide.html#cargotoml-vs-cargolock
[2]: http://doc.crates.io/faq.html#why-do-binaries-have-cargolock-in-version-control-but-not-libraries

Author: gifnksm

.gitignore

@@ -9,6 +9,7 @@
 
 # Generated by Cargo
 /target/
+Cargo.lock
 
 # Fixture files
 tests/fixtures/merge/*.gcda