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