travis: add darwin build environment

This adds a build environment based on osx, so we can ensure it builds
on this platform as well.

Signed-off-by: Sean Cross <sean@xobs.io>

Author: xobs

.travis.yml

@@ -1,22 +1,28 @@
 language: rust
-rust:
-    - stable
-    - beta
-    - nightly
-matrix:
+jobs:
   allow_failures:
     - rust: nightly
-before_install:
-    - sudo apt-get -qq update
-    - sudo apt-get install -y cmake
-    - sudo apt-get install -y libgtk-3-dev
+  include:
+    - env: TEST_CMD=xvfb-run
+      rust: nightly
+    - env: TEST_CMD=xvfb-run
+      rust: beta
+    - env: TEST_CMD=xvfb-run
+      rust: stable
+    - env: MACOSX_DEPLOYMENT_TARGET=10.7 SKIP_INSTALL=1
+      rust: stable
+      os: osx
+before_install: |
+  [ "x$SKIP_INSTALL" != "x" ] || sudo apt-get -qq update
+  [ "x$SKIP_INSTALL" != "x" ] || sudo apt-get install -y libgtk-3-dev
+
 script:
     - cd ui-sys
     - cargo build --verbose --tests --examples
-    - xvfb-run cargo test
+    - $TEST_CMD cargo test
     - cd ../iui
     - cargo build --verbose --tests --examples
-    - xvfb-run cargo test
+    - $TEST_CMD cargo test
 after_success: |
   [ $TRAVIS_BRANCH = master ] &&
   [ $TRAVIS_PULL_REQUEST = false ] &&