Pin minimum support at Rust 1.33

No promises about maintaining that, just want to know if/when I do something that breaks it

.travis.yml

@@ -4,6 +4,8 @@ jobs:
   include:
   - rust: stable
     os: linux
+  - rust: 1.33.0
+    os: linux
   - rust: stable
     os: osx
   - rust: stable-msvc

aeron-driver-sys/build.rs

@@ -93,7 +93,7 @@ pub fn main() {
 
     println!("cargo:include={}", header_path.display());
     let bindings = bindgen::Builder::default()
-        .clang_arg(&format!("-I{}", header_path.display()))
+        .clang_arg(format!("-I{}", header_path.display()))
         .header("bindings.h")
         .whitelist_function("aeron_.*")
         .whitelist_type("aeron_.*")