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

HyprMXMobile/OHHTTPStubs,tarbrain/OHHTTPStubs,danielctull-forks/OHHTTPStubs,tarbrain/OHHTTPStubs,danielctull-forks/OHHTTPStubs,HyprMXMobile/OHHTTPStubs,danielctull-forks/OHHTTPStubs,tarbrain/OHHTTPStubs,HyprMXMobile/OHHTTPStubs,HyprMXMobile/OHHTTPStubs,danielctull-forks/OHHTTPStubs,tarbrain/OHHTTPStubs