fix: downgrade os_x image to build succesffully

Author: siddharthlatest

.travis.yml

@@ -45,7 +45,7 @@ jobs:
 
     - name: macos
       os: osx
-      osx_image: xcode10.2
+      osx_image: xcode9.4
 
       before_install:
       - curl -LO https://github.com/neo4j-drivers/seabolt/releases/download/v1.7.4/seabolt-1.7.4-Darwin.tar.gz