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
@@ -28,9 +20,3 @@ before_install:
   - if test "$TRAVIS_OS_NAME" == "osx"; then brew update; brew install openssl; ln -s /usr/local/opt/openssl/lib/libcrypto.1.0.0.dylib /usr/local/lib/; ln -s /usr/local/opt/openssl/lib/libssl.1.0.0.dylib /usr/local/lib/; fi
 script:
   - ./build.sh
-notifications:
-  webhooks:
-    secure: "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"
-  on_success: always
-  on_failure: always
-  on_start: always