ci: Test on stable and beta, as well as all released versions

refs nix-rust#238

Author: kamalmarhubi

.travis.yml

@@ -2,8 +2,20 @@
 sudo: false
 language: rust
 rust:
-  - nightly
+  - 1.0.0
   - 1.1.0
+  - 1.2.0
+  - 1.3.0
+  - 1.4.0
+  - 1.5.0
+  - stable
+  - beta
+  - nightly
+
+# Failures on nightly shouldn't fail the overall build.
+matrix:
+  allow_failures:
+    - rust: nightly
 
 os:
   - linux