Run UI tests only on nightly.

ui/Cargo.toml

@@ -13,3 +13,4 @@ path = "main.rs"
 compiletest_rs = { version = "0.3", features = ["stable"] }
 typed-html = { path = "../typed-html" }
 typed-html-macros = { path = "../macros" }
+version_check = "0.1.5"

ui/main.rs

@@ -1,5 +1,11 @@
 #[test]
 fn ui() {
+    extern crate version_check;
+
+    if !version_check::is_nightly().unwrap_or(false) {
+        return;
+    }
+
     extern crate compiletest_rs as compiletest;
 
     let mut config = compiletest::Config {