Fix travis.yml.

.travis.yml

@@ -1,9 +1,20 @@
+sudo: false
 language: rust
-
+matrix:
+  include:
+    - rust: nightly
+      env: TEST_SUITE=suite_nightly
+    - rust: beta
+      env: TEST_SUITE=suite_beta
+    - rust: stable
+      env: TEST_SUITE=suite_stable
+  allow_failures:
+    - rust: nightly
+    - rust: beta
 script:
     - rustc --version
     - cargo --version
     - cargo build --verbose
     - cargo build --verbose --features arbitrary
     - cargo build --verbose --features serde-serialize
-    - cargo test --verbose --features arbitrary serde-serialize
+    - cargo test --verbose --features "arbitrary serde-serialize"