Xcode 8.3 and Ubuntu 14.04

This release adds Travis CI of Linux builds on Ubuntu 14.04. macOS builds are verified with Xcode 8.3. Future commits must maintain compatibility with both macOS and Linux.

Run the ubuntu.sh script in a bare Ubuntu instance to install Linux dependencies. libobjc2, gnustep-make, and gnustep-base are vendored from forks in http://github.com/nulang.