Fix running roundtrip tests against dev build of rustc

The version printed by `rustc +dev --version` in fact ends
with "-dev\n", not "-dev".

build.rs

@@ -38,6 +38,6 @@ fn rustc_version() -> Option<Compiler> {
         return None;
     }
     let minor = pieces.next()?.parse().ok()?;
-    let nightly = version.contains("nightly") || version.ends_with("-dev");
+    let nightly = version.contains("nightly") || version.trim_end().ends_with("-dev");
     Some(Compiler { minor, nightly })
 }