PROTON-2013: Updated Travis CI macOS jobs to be xcode8.3 and xcode10.1
PROTON-2013: Updated Travis CI macOS jobs to be xcode8.3 and xcode10.1

(cherry picked from commit 104c3e260f7386ef6cab599fc73d20c65d25f86d)

apache/qpid-proton,ssorj/qpid-proton,apache/qpid-proton,kgiusti/qpid-proton,astitcher/qpid-proton,gemmellr/qpid-proton,ChugR/qpid-proton,kgiusti/qpid-proton,ChugR/qpid-proton,kgiusti/qpid-proton,astitcher/qpid-proton,kgiusti/qpid-proton,ssorj/qpid-proton,astitcher/qpid-proton,gemmellr/qpid-proton,ssorj/qpid-proton,kgiusti/qpid-proton,kgiusti/qpid-proton,ssorj/qpid-proton,gemmellr/qpid-proton,ChugR/qpid-proton,astitcher/qpid-proton,gemmellr/qpid-proton,apache/qpid-proton,ssorj/qpid-proton,apache/qpid-proton,ChugR/qpid-proton,ssorj/qpid-proton,astitcher/qpid-proton,ChugR/qpid-proton,ChugR/qpid-proton,apache/qpid-proton,astitcher/qpid-proton,gemmellr/qpid-proton,gemmellr/qpid-proton,apache/qpid-proton