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.

macosx/brew-deps.sh

@@ -0,0 +1,69 @@
+#!/bin/sh
+
+#
+# Install Homebrew dependencies
+#
+# This script handles build dependencies other than those provided by
+# MacOS and Xcode, for a Mosh build using macosx/build.sh or the
+# native autoconf/automake build for CI.  It is intended to be used by
+# a build system, and should be agnostic to any particular system.
+#
+# Similar scripts could be developed for MacPorts, direct dependency
+# builds, etc.
+#
+
+#
+# Install and/or configure the system used to provide dependencies.
+#
+install()
+{
+    # Straight from https://brew.sh
+    if ! brew --version > /dev/null 2>&1; then
+	/usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
+    fi
+}
+
+#
+# Install up-to-date build dependencies required for a development or
+# CI build.  These dependencies only need to provide runtime
+# dependencies for the build system, support for things like previous
+# OS versions and fat binaries is not needed.
+#
+deps()
+{
+    brew update
+    brew update
+    brew reinstall tmux
+    brew reinstall protobuf
+}
+
+#
+# Install build dependencies required for the MacOS package build.
+# Runtime dependencies are required to support the targeted OS X
+# version, static libraries, and fat binaries for the package build.
+#
+# This reinstalls protobuf with --universal --bottle to get a fat
+# library that will run on any machine.  (This takes about 15 minutes
+# on current Travis infrastructure.)
+#
+package_deps()
+{
+    deps
+    brew rm protobuf
+    brew install protobuf --universal --bottle
+}
+
+#
+# Describe the dependencies installed and used as best as possible.
+#
+describe()
+{
+    brew --version > brew-version.txt
+    brew info --json=v1 --installed > brew-info.json
+}
+
+#
+# Do something.
+#
+set -e
+"$@"