Update travis to build on all three major platforms

Utilizes the automatic build matrix to build with both wanted Rust
versions on all platforms. Builds without default features, and on
nightly with minimal versions are only ran on the Linux platform.

.travis.yml

@@ -1,10 +1,15 @@
 language: rust
+os:
+  - linux
+  - osx
+  - windows
+rust:
+  - stable
+  - nightly
 matrix:
   include:
-    - rust: stable
     - rust: stable
       env: FLAGS="--no-default-features"
-    - rust: nightly
     - rust: nightly
       env: FLAGS="-Z minimal-versions"
 script: