Add macOS Sierra to the travis build and bump Ruby versions

Author: ignisf

.travis.yml

@@ -1,34 +1,37 @@
+language: ruby
 rvm:
-  - 2.3.1
+  - 2.3.2
   - 2.2
   - 2.1
-  - rbx
+  - rbx-2.2.7
 matrix:
   include:
-    - rvm: 2.2
+    - rvm: 2.3.2
       os: osx
-      osx_image: xcode8
+      osx_image: xcode8.2
     - rvm: 2.2
       os: osx
       osx_image: xcode7.3
-    - rvm: 2.2
-      os: osx
-      osx_image: xcode7.1
     - rvm: system
       os: osx
-      osx_image: xcode7.1
+      osx_image: xcode7.3
     - rvm: system
       os: osx
-      osx_image: xcode7.3
+      osx_image: xcode8.2
+    - rvm: 2.3.2
+      os: osx
+      osx_image: xcode6.4
     - rvm: system
       os: osx
-      osx_image: xcode8
-    - rvm: ruby-2.3.1-clang
+      osx_image: xcode6.4
+    - rvm: ruby-2.3.2-clang
       env: CXX=clang++
     - rvm: 1.9.3
   allow_failures:
     - rvm: 1.9.3
-    - rvm: rbx
+    - rvm: rbx-2.2.7
+    - os: osx
+      osx_image: xcode6.4
   fast_finish: true
 bundler_args: --jobs=1 --retry=3
 before_install: