leveldb: Update Travis CI configuration for open source build.

This aligns the Travis CI configuration with google/crc32c and
google/snappy, to simplify maintenance.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=188547648

.travis.yml

@@ -1,13 +1,54 @@
-language: cpp
-compiler:
-- clang
-- gcc
-os:
-- linux
-- osx
+# Build matrix / environment variable are explained on:
+# http://about.travis-ci.org/docs/user/build-configuration/
+# This file can be validated on: http://lint.travis-ci.org/
+
 sudo: false
-before_install:
-- echo $LANG
-- echo $LC_ALL
+dist: trusty
+language: cpp
+
+compiler:
+  - gcc
+  - clang
+os:
+  - linux
+  - osx
+
+addons:
+  apt:
+    # List of whitelisted in travis packages for ubuntu-trusty can be found here:
+    #   https://github.com/travis-ci/apt-package-whitelist/blob/master/ubuntu-trusty
+    # List of whitelisted in travis apt-sources:
+    #   https://github.com/travis-ci/apt-source-whitelist/blob/master/ubuntu.json
+    sources:
+    - ubuntu-toolchain-r-test
+    - llvm-toolchain-trusty-5.0
+    packages:
+    - gcc-7
+    - g++-7
+    - clang-5.0
+    - libgoogle-perftools-dev
+    - libkyotocabinet-dev
+    - libsnappy-dev
+    - libsqlite3-dev
+
+install:
+# Travis doesn't have a DSL for installing homebrew packages yet. Status tracked
+# in https://github.com/travis-ci/travis-ci/issues/5377
+# The Travis VM image for Mac already has a link at /usr/local/include/c++,
+# causing Homebrew's gcc@7 installation to error out. This was reported to
+# Homebrew maintainers at https://github.com/Homebrew/brew/issues/1742 and
+# removing the link emerged as a workaround.
+- if [ "$TRAVIS_OS_NAME" == "osx" ]; then
+    brew update;
+    if [ -L /usr/local/include/c++ ]; then rm /usr/local/include/c++; fi;
+    brew install gcc@7;
+    brew install crc32c gperftools kyoto-cabinet snappy sqlite3;
+  fi
+# /usr/bin/gcc is stuck to old versions on both Linux and OSX.
+- if [ "$CXX" = "g++" ]; then export CXX="g++-7" CC="gcc-7"; fi
+- echo ${CC}
+- echo ${CXX}
+- ${CXX} --version
+
 script:
 - make -j 4 check