Add dependencies for top-level build script

Helps avoid unnecessarily compiling the main library!

Author: alexcrichton

build.rs

@@ -8,6 +8,8 @@ use dotenv::dotenv;
 use std::env;
 
 fn main() {
+    println!("cargo:rerun-if-env-changed=TEST_DATABASE_URL");
+    println!("cargo:rerun-if-changed=build.rs");
     if env::var("PROFILE") == Ok("debug".into()) {
         let _ = dotenv();
         if let Ok(database_url) = env::var("TEST_DATABASE_URL") {