Don't use cross for thumb target CI

Author: Amanieu

.travis.yml

@@ -33,7 +33,7 @@ matrix:
     - name: "x86_64-unknown-linux-gnu (stable)"
       rust: stable
       env: TARGET=x86_64-unknown-linux-gnu
-    - name: "x86_64-unknown-linux-gnu (Rust 1.31.0)"
+    - name: "x86_64-unknown-linux-gnu (Rust 1.32.0)"
       rust: 1.32.0
       env: TARGET=x86_64-unknown-linux-gnu
     - name: "i686-unknown-linux-gnu"
@@ -64,7 +64,11 @@ matrix:
 
     # Ensure that we successfully build without libstd
     - name: "thumbv6m-none-eabi"
-      env: TARGET=thumbv6m-none-eabi CROSS=1 NO_STD=1
+      env: TARGET=thumbv6m-none-eabi NO_STD=1
+      script:
+        # cross doesn't seem to work with thumb targets...
+        - rustup target install $TARGET
+        - sh ci/run.sh
 
 install: travis_retry rustup target add "${TARGET}"
 script: sh ci/run.sh