Have travis use a newer image for the OSX build (#10034)
Have travis use a newer image for the OSX build (#10034)

Suggested by travis support for stopping the randomly-halting-builds issue.
thr0w/Thr0wScript,donaldpipowitch/TypeScript,alexeagle/TypeScript,vilic/TypeScript,SaschaNaz/TypeScript,nojvek/TypeScript,kitsonk/TypeScript,mihailik/TypeScript,chuckjaz/TypeScript,Eyas/TypeScript,nojvek/TypeScript,synaptek/TypeScript,vilic/TypeScript,Eyas/TypeScript,mihailik/TypeScript,jwbay/TypeScript,microsoft/TypeScript,jwbay/TypeScript,jwbay/TypeScript,Eyas/TypeScript,kpreisser/TypeScript,weswigham/TypeScript,mihailik/TypeScript,kpreisser/TypeScript,RyanCavanaugh/TypeScript,basarat/TypeScript,erikmcc/TypeScript,nojvek/TypeScript,Microsoft/TypeScript,chuckjaz/TypeScript,DLehenbauer/TypeScript,SaschaNaz/TypeScript,kpreisser/TypeScript,DLehenbauer/TypeScript,donaldpipowitch/TypeScript,erikmcc/TypeScript,minestarks/TypeScript,basarat/TypeScript,alexeagle/TypeScript,basarat/TypeScript,erikmcc/TypeScript,Microsoft/TypeScript,minestarks/TypeScript,chuckjaz/TypeScript,mihailik/TypeScript,erikmcc/TypeScript,RyanCavanaugh/TypeScript,nojvek/TypeScript,microsoft/TypeScript,weswigham/TypeScript,thr0w/Thr0wScript,kitsonk/TypeScript,vilic/TypeScript,Eyas/TypeScript,TukekeSoft/TypeScript,donaldpipowitch/TypeScript,thr0w/Thr0wScript,kitsonk/TypeScript,SaschaNaz/TypeScript,synaptek/TypeScript,jeremyepling/TypeScript,DLehenbauer/TypeScript,minestarks/TypeScript,jeremyepling/TypeScript,RyanCavanaugh/TypeScript,synaptek/TypeScript,vilic/TypeScript,microsoft/TypeScript,weswigham/TypeScript,Microsoft/TypeScript,DLehenbauer/TypeScript,thr0w/Thr0wScript,basarat/TypeScript,jwbay/TypeScript,SaschaNaz/TypeScript,synaptek/TypeScript,TukekeSoft/TypeScript,TukekeSoft/TypeScript,jeremyepling/TypeScript,chuckjaz/TypeScript,donaldpipowitch/TypeScript,alexeagle/TypeScript