Use build matrix on Travis CI

This makes it easy to see which target failed to build.

.travis.yml

 language: rust
+env:
+  -
+  - TARGET=aarch64-unknown-linux-gnu
+  - TARGET=x86_64-unknown-redox
 rust:
   - nightly
 cache: cargo
 before_script:
   - rustup component add rustfmt-preview
-  - rustup target add x86_64-unknown-redox
-  - rustup target add aarch64-unknown-linux-gnu
+  - if [ -n "$TARGET" ]; then rustup target add $TARGET; fi
 script:
-  - bash ./ci.sh
+  - ./fmt.sh -- --write-mode=diff
+  - cargo build $([ -n "$TARGET" ] && echo --target="$TARGET")
+  - if [ -z "$TARGET" ]; then ./test.sh; fi
 notifications:
   email: false

ci.sh

-#!/bin/bash
-set -ex
-
-./fmt.sh -- --write-mode=diff
-./test.sh
-cargo build --target=x86_64-unknown-redox
-if [ $(arch) == "x86_64" ]
-then
-    cargo build --target=aarch64-unknown-linux-gnu
-else
-    cargo build --target=x86_64-unknown-linux-gnu
-fi