Merge pull request #393 from tzemanovic/tomas/compile-fail-must-be-debug

fix compile fail test: must be debug

proptest-derive/tests/compile-fail/must-be-debug.rs

@@ -11,5 +11,5 @@ extern crate proptest_derive;
 
 fn main() {}
 
-#[derive(Arbitrary)]
+#[derive(Arbitrary)] //~ `Foo` doesn't implement `Debug` [E0277]
 struct Foo { x: usize } //~ `Foo` doesn't implement `Debug` [E0277]