Remove testing against XCode 9.2

Upstream setuptools, pip and wheel yields dyld: lazy symbol binding failed: Symbol not found: _utimensat error

Author: Trung Lê

.travis.yml

@@ -10,9 +10,6 @@ matrix:
     - rvm: 2.5.1
       os: osx
       osx_image: xcode9.4.1
-    - rvm: 2.5.1
-      os: osx
-      osx_image: xcode9.2
   allow_failures:
     - rvm: rubinius-3
   fast_finish: true
@@ -25,7 +22,6 @@ bundler_args: --jobs=4 --retry=3
 before_install:
   - if [ "$TRAVIS_OS_NAME" == "linux" ]; then gem update bundler; fi
   - if [ "$TRAVIS_OS_NAME" == "linux" ]; then gem update --system; fi
-  - if [ "$TRAVIS_OS_NAME" == "osx"]; then xcode-select --install; fi
   - if [ "$TRAVIS_OS_NAME" == "osx" -a "$TRAVIS_RUBY_VERSION" == "system" ]; then sudo gem install bundler; fi
   - if [ "$TRAVIS_OS_NAME" == "osx" -a "$TRAVIS_RUBY_VERSION" != "system" ]; then gem update --system; fi
   - if [ "$TRAVIS_OS_NAME" == "osx" -a "$TRAVIS_RUBY_VERSION" != "system" ]; then gem update bundler; fi