Update Travis CI configuration.

The Travis CI configuration updates reflect the following changes:
* Container-based builds (sudo: false) have been removed.
  https://changelog.travis-ci.com/the-container-based-build-environment-is-fully-deprecated-84517
* Ubuntu Xenial (16.04) is available as a base image.
  https://blog.travis-ci.com/2018-11-08-xenial-release
* Homebrew now has a dedicated DSL.
  https://docs.travis-ci.com/user/installing-dependencies/#installing-packages-on-os-x

To take full advantage of VM resources, CI builds now use Ninja
https://ninja-build.org/ instead of Make.

.travis.yml

@@ -2,8 +2,7 @@
 # http://about.travis-ci.org/docs/user/build-configuration/
 # This file can be validated on: http://lint.travis-ci.org/
 
-sudo: false
-dist: trusty
+dist: xenial
 language: cpp
 
 compiler:
@@ -31,26 +30,28 @@ addons:
     # List of whitelisted in travis apt-sources:
     #   https://github.com/travis-ci/apt-source-whitelist/blob/master/ubuntu.json
     sources:
+    - llvm-toolchain-xenial-7
     - ubuntu-toolchain-r-test
-    - llvm-toolchain-trusty-6.0
     packages:
+    - clang-7
     - cmake
     - gcc-8
     - g++-8
-    - clang-6.0
+    - ninja-build
+  homebrew:
+    packages:
+    - cmake
+    - gcc@8
+    - ninja
 
-install:
-# Travis doesn't have a DSL for installing homebrew packages yet. Status tracked
-# in https://github.com/travis-ci/travis-ci/issues/5377
+before_install:
 # The Travis VM image for Mac already has a link at /usr/local/include/c++,
-# causing Homebrew's gcc@7 installation to error out. This was reported to
+# causing Homebrew's gcc installation to error out. This was reported to
 # Homebrew maintainers at https://github.com/Homebrew/brew/issues/1742 and
 # removing the link emerged as a workaround.
-- if [ "$TRAVIS_OS_NAME" == "osx" ]; then
-    brew update;
-    if [ -L /usr/local/include/c++ ]; then rm /usr/local/include/c++; fi;
-    brew install gcc@8;
-  fi
+- if [ "$TRAVIS_OS_NAME" == "osx" ]; then rm -f /usr/local/include/c++ ; fi
+
+install:
 # /usr/bin/gcc is stuck to old versions on both Linux and OSX.
 - if [ "$CXX" = "g++" ]; then export CXX="g++-8" CC="gcc-8"; fi
 - echo ${CC}
@@ -60,7 +61,7 @@ install:
 
 before_script:
 - mkdir -p build && cd build
-- cmake .. -DCMAKE_BUILD_TYPE=$BUILD_TYPE -DSNAPPY_REQUIRE_AVX=ON
+- cmake .. -G Ninja -DCMAKE_BUILD_TYPE=$BUILD_TYPE -DSNAPPY_REQUIRE_AVX=ON
 - cmake --build .
 - cd ..