Update Travis configuration to build with Xcode 7.3
Update Travis configuration to build with Xcode 7.3

Travis retired the Xcode 7.2 image and has been routing jobs to the Xcode 7.3 image.

mattrubin/SwiftGit2,SwiftGit2/SwiftGit2,mattrubin/SwiftGit2,SwiftGit2/SwiftGit2