Update travis.ml to use Xcode7 image
Update travis.ml to use Xcode7 image

SummerHanada/OHHTTPStubs,oenius/OHHTTPStubs,virajs/OHHTTPStubs,SummerHanada/OHHTTPStubs,SummerHanada/OHHTTPStubs,oenius/OHHTTPStubs,hirohisa/OHHTTPStubs,hirohisa/OHHTTPStubs,tarbrain/OHHTTPStubs,hirohisa/OHHTTPStubs,virajs/OHHTTPStubs,virajs/OHHTTPStubs,tarbrain/OHHTTPStubs,tarbrain/OHHTTPStubs,hirohisa/OHHTTPStubs,tarbrain/OHHTTPStubs,oenius/OHHTTPStubs,oenius/OHHTTPStubs,virajs/OHHTTPStubs,SummerHanada/OHHTTPStubs