Bump libgit2 in version probe and readme (#808)

Author: dtolnay

Diff for: README.md

@@ -16,7 +16,7 @@ stable release as well.
 
 ## Version of libgit2
 
-Currently this library requires libgit2 1.3.0. The source for libgit2 is
+Currently this library requires libgit2 1.4.0. The source for libgit2 is
 included in the libgit2-sys crate so there's no need to pre-install the libgit2
 library, the libgit2-sys crate will figure that and/or build that for you.
 

Diff for: libgit2-sys/build.rs

@@ -14,7 +14,7 @@ fn main() {
     let try_to_use_system_libgit2 = !vendored && !zlib_ng_compat;
     if try_to_use_system_libgit2 {
         let mut cfg = pkg_config::Config::new();
-        if let Ok(lib) = cfg.atleast_version("1.3.0").probe("libgit2") {
+        if let Ok(lib) = cfg.atleast_version("1.4.0").probe("libgit2") {
             for include in &lib.include_paths {
                 println!("cargo:root={}", include.display());
             }