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: "V+ynhkZnRWn9VemSgBKI6KB5WxSE/SuoIKFnBQFb/1O0r15aac4/VyzuRdTK8V29OxdI1vK0J3UET2c2v2Q2IusFc4rCHGcRIobjHq+YW8944betuYfHkpVgniHofaGLgJH8gdlZv4elDdVHWKndYIGTaBU8+dO8ktp+9BFGYhs="
-  on_success: always
-  on_failure: always
-  on_start: always