Update Travis to macOS Sierra

[skip appveyor]

.travis.yml

@@ -1,15 +1,6 @@
 language: csharp
-sudo: required
+sudo: false
 dist: trusty
-addons:
-  apt:
-    packages:
-    - gettext
-    - libcurl4-openssl-dev
-    - libicu-dev
-    - libssl-dev
-    - libunwind8
-    - zlib1g
 env:
   global:
     - DOTNET_SKIP_FIRST_TIME_EXPERIENCE: true
@@ -18,6 +9,7 @@ mono: none
 os:
   - linux
   - osx
+osx_image: xcode8.2
 branches:
   only:
     - master