Fix some doc tests

Author: alexcrichton

src/config.rs

@@ -214,6 +214,7 @@ impl Config {
     /// # Example
     ///
     /// ```
+    /// # #![allow(unstable)]
     /// use git2::Config;
     ///
     /// let cfg = Config::new().unwrap();

src/lib.rs

@@ -23,6 +23,7 @@
 //! already exist.
 //!
 //! ```no_run
+//! # #![allow(unstable)]
 //! use git2::Repository;
 //!
 //! let path = Path::new("/path/to/a/repo");
@@ -35,6 +36,7 @@
 //! ### Opening an existing repository
 //!
 //! ```no_run
+//! # #![allow(unstable)]
 //! use git2::Repository;
 //!
 //! let path = Path::new("/path/to/a/repo");
@@ -47,6 +49,7 @@
 //! ### Cloning an existing repository
 //!
 //! ```no_run
+//! # #![allow(unstable)]
 //! use git2::Repository;
 //!
 //! let url = "https://github.com/alexcrichton/git2-rs";