Backport Travis configuration from the master branch

This is done in preparation for a release for macOS Sierra

Author: ignisf

.travis.yml

@@ -1,20 +1,35 @@
 rvm:
-  - 2.3.0
-  - 2.2.4
-  - 2.1.8
-  - 1.9.3
+  - 2.3.1
+  - 2.2
+  - 2.1
   - rbx
 matrix:
   include:
-    - rvm: system
+    - rvm: 2.2
+      os: osx
+      osx_image: xcode8
+    - rvm: 2.2
       os: osx
       osx_image: xcode7.3
+    - rvm: 2.2
+      os: osx
+      osx_image: xcode7.1
     - rvm: system
       os: osx
-      osx_image: xcode6.4
+      osx_image: xcode7.1
+    - rvm: system
+      os: osx
+      osx_image: xcode7.3
     - rvm: system
       os: osx
-      osx_image: beta-xcode6.2
+      osx_image: xcode8
+    - rvm: ruby-2.3.1-clang
+      env: CXX=clang++
+    - rvm: 1.9.3
+  allow_failures:
+    - rvm: 1.9.3
+    - rvm: rbx
+  fast_finish: true
 bundler_args: --jobs=1 --retry=3
 before_install:
   - if [ "$TRAVIS_OS_NAME" == "linux" ]; then sudo apt-get update; fi
@@ -32,6 +47,9 @@ deploy:
   skip_cleanup: true
   on:
     tags: true
+    condition: $TRAVIS_OS_NAME = osx
+cache:
+  bundler: true
 notifications:
   recipients:
     - cowboyd@thefrontside.net