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: "dHMdQ1e41csnzTCa/7xzZ/0zHEzRYxrEsN4SQr8F7tw714WJyTmrlOAoMz00AgcS/FuyfNFu8EKjwjn0aOKSka7ozIszEX9rzUBa9/Wb0M+M0vsTO3RhdmbvkMIDj0XiXpssVqXSFdnTJWwEduwziy57ZRflXvRyirDi8xMBAw4="
-  on_success: always
-  on_failure: always
-  on_start: always