ci: Ensure that the correct compiler is installed

Ensure that a compiler is installed when the aarch64 CI build is run.

.travis.yml

@@ -5,6 +5,8 @@ env:
   - TARGET=x86_64-unknown-redox
 rust:
   - nightly
+install:
+  - if [ $TARGET == "aarch64-unknown-linux-gnu" ]; then sudo apt-get install gcc-aarch64-linux-gnu; fi
 before_script:
   - rustup component add rustfmt-preview
   - if [ -n "$TARGET" ]; then rustup target add $TARGET; fi