Fix type `mode_t` type errors after updating bindings (#78)

Signed-off-by: Victor <victor.porof@gmail.com>

src/environment.rs

@@ -406,7 +406,7 @@ impl EnvironmentBuilder {
     ///
     /// The path may not contain the null character, Windows UNC (Uniform Naming Convention)
     /// paths are not supported either.
-    pub fn open_with_permissions(&self, path: &Path, mode: ffi::mode_t) -> Result<Environment> {
+    pub fn open_with_permissions(&self, path: &Path, mode: ffi::mdb_mode_t) -> Result<Environment> {
         let mut env: *mut ffi::MDB_env = ptr::null_mut();
         unsafe {
             lmdb_try!(ffi::mdb_env_create(&mut env));