Use xcode6.4 image per documentation at https://docs.travis-ci.com/user/osx-ci-environment/#OS-X-Version
Use xcode6.4 image per documentation at https://docs.travis-ci.com/user/osx-ci-environment/#OS-X-Version

Fixes #1209

rcrowder/nupic.core,ywcui1990/nupic.core,neuroidss/nupic.core,numenta/nupic.core,subutai/nupic.core,ywcui1990/nupic.core,numenta/htmresearch-core,numenta/htmresearch-core,lscheinkman/nupic.core,lscheinkman/nupic.core,neuroidss/nupic.core,EricSB/nupic.core,numenta/nupic.core,subutai/nupic.core,neuroidss/nupic.core,rcrowder/nupic.core,EricSB/nupic.core,EricSB/nupic.core,rhyolight/nupic.core,numenta/nupic.core,breznak/nupic.core,breznak/nupic.core,rcrowder/nupic.core,ywcui1990/nupic.core,ywcui1990/nupic.core,rcrowder/nupic.core,lscheinkman/nupic.core,numenta/htmresearch-core,numenta/htmresearch-core,subutai/nupic.core,scottpurdy/nupic.core,breznak/nupic.core,scottpurdy/nupic.core,rhyolight/nupic.core,breznak/nupic.core,lscheinkman/nupic.core,scottpurdy/nupic.core,EricSB/nupic.core,rhyolight/nupic.core,rhyolight/nupic.core,scottpurdy/nupic.core,neuroidss/nupic.core,numenta/nupic.core,subutai/nupic.core