travis: env correctly

.travis.yml

@@ -8,14 +8,11 @@ cache:
 
 os:
   - linux:
-      - env:
-        - TARGET=x86_64-unknown-linux-gnu
+    env: TARGET=x86_64-unknown-linux-gnu
   - osx:
-    - env:
-      - TARGET=x86_64-apple-darwin
+    env: TARGET=x86_64-apple-darwin
   - windows:
-      - env:
-        - TARGET=x86_64-pc-windows-msvc
+    env: TARGET=x86_64-pc-windows-msvc
 
 before_script:
   - rustup component add rustfmt