travis: test rust version before rustup

.travis.yml

@@ -56,8 +56,9 @@ install:
   # Install Rust.
   # TODO(ry) Include rustc in third_party.
   # https://github.com/denoland/deno/issues/386
-  if [ ! $(which rustc) ]; then
+  if [ ! $(rustc --version | grep 1.30.0) ]; then
     curl -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain 1.30.0
+    rustup default 1.30.0
   fi
   rustc --version
   cargo --version