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 +20,4 @@ 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