Install bundler v2.0.1 before run test with Xcode10.1 image on travis CI

Author: ra1028

.travis.yml

@@ -31,6 +31,7 @@ matrix:
   include:
     - <<: *testing_xcode
       osx_image: xcode10.1
+      install: gem install bundler -v 2.0.1
 
     - <<: *testing_xcode
       osx_image: xcode10.2
@@ -48,4 +49,4 @@ matrix:
       env: SWIFT_VERSION=5.0
 
 notifications:
-  email: false
+  email: false