Add support for 32-bit

Author: chansuke

.travis.yml

@@ -32,3 +32,9 @@ env:
   global:
     # Override the default `--features unstable` used for the nightly branch
     - TRAVIS_CARGO_NIGHTLY_FEATURE=""
+    - ARCH=x86_64
+    - ARCH=i686
+
+script:
+  - curl -sSL https://raw.githubusercontent.com/carllerche/travis-rust-matrix/master/test | bash
+