Update CI configurations.

Bump GCC and Clang on Travis and remove Visual Studio 2015 from AppVeyor.

.appveyor.yml

@@ -11,9 +11,6 @@ environment:
     - JOB: Visual Studio 2017
       APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
       CMAKE_GENERATOR: Visual Studio 15 2017
-    - JOB: Visual Studio 2015
-      APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
-      CMAKE_GENERATOR: Visual Studio 14 2015
 
 platform:
   - x86
@@ -38,4 +35,4 @@ build_script:
   - cd ..
 
 test_script:
-  - out\%CONFIGURATION%\snappy_unittest
+  - out\%CONFIGURATION%\snappy_unittest

.travis.yml

@@ -30,27 +30,27 @@ addons:
     #   https://github.com/travis-ci/apt-source-whitelist/blob/master/ubuntu.json
     sources:
     - ubuntu-toolchain-r-test
-    - llvm-toolchain-trusty-4.0
+    - llvm-toolchain-trusty-5.0
     packages:
     - cmake
-    - gcc-6
-    - g++-6
-    - clang-4.0
+    - gcc-7
+    - g++-7
+    - clang-5.0
 
 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@6 installation to error out. This was reported to
+# 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 [ "$TRAVIS_OS_NAME" == "osx" ]; then
     brew update;
     if [ -L /usr/local/include/c++ ]; then rm /usr/local/include/c++; fi;
-    brew install gcc@6;
+    brew install gcc@7;
   fi
-# /usr/bin/gcc is stuck to old versions by on both Linux and OSX.
-- if [ "$CXX" = "g++" ]; then export CXX="g++-6" CC="gcc-6"; 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
@@ -63,4 +63,4 @@ before_script:
 - cd ..
 
 script:
-- build/snappy_unittest
+- build/snappy_unittest