OS X package build on Travis

This builds an OS X package and deploys it to a GitHub release when a
tag is pushed.  It also generates a tarball reporting the build
environment and configuration.  The build log is still separate.

This is not yet final, it deploys to cgull/mosh and not
mobile-shell/mosh.

It should not affect Linux or OS X CI builds (other than the change to
the Travis Xcode 7.1 image).

Included changes:

The Travis Xcode 7 image seems to have added tmux while we were gone,
breaking our Homebrew setup.

There seems to be no clean reliable way to determine whether a
Homebrew package is installed or needs updating.  Reinstalling is less
efficient but seems to work reliably.

The OS X build is now split between four files:

.travis.yml contains Travis-specific CI/release build configuration.

macosx/brew.sh contains Homebrew-specific package manager installs and
reporting.

macosx/osx-xcode.sh contains Apple-specific OS/X and Xcode reporting.

macosx/build.sh does the actual package build.