Add more platforms to CI

 * aarch64-unknown-linux-musl
 * sparc64-unknown-linux-gnu
 * x86_64-unknown-linux-gnux32

These don't pass yet, but should be supportable by `nix`, so let's add them now so we can move towards fully supporting them.

.travis.yml

@@ -49,6 +49,8 @@ matrix:
     # Linux
     - env: TARGET=aarch64-unknown-linux-gnu
       rust: 1.24.1
+    - env: TARGET=aarch64-unknown-linux-musl
+      rust: 1.24.1
     - env: TARGET=arm-unknown-linux-gnueabi
       rust: 1.24.1
     - env: TARGET=arm-unknown-linux-musleabi DISABLE_TESTS=1
@@ -75,8 +77,12 @@ matrix:
       rust: 1.24.1
     - env: TARGET=s390x-unknown-linux-gnu DISABLE_TESTS=1
       rust: 1.24.1
+    - env: TARGET=sparc64-unknown-linux-gnu
+      rust: 1.24.1
     - env: TARGET=x86_64-unknown-linux-gnu
       rust: 1.24.1
+    - env: TARGET=x86_64-unknown-linux-gnux32
+      rust: 1.24.1
     - env: TARGET=x86_64-unknown-linux-musl
       rust: 1.24.1
 
@@ -93,6 +99,14 @@ matrix:
     - env: TARGET=x86_64-unknown-linux-gnu
       rust: stable
 
+  allow_failures:
+    - env: TARGET=aarch64-unknown-linux-musl
+      rust: 1.24.1
+    - env: TARGET=sparc64-unknown-linux-gnu
+      rust: 1.24.1
+    - env: TARGET=x86_64-unknown-linux-gnux32
+      rust: 1.24.1
+
 before_install: set -e
 
 install: