Create .travis.yml

Basic Travis CI setup.  I don't expect it to work because cross-compiling doesn't have access to Boost.
Added missing apt update step

Also removed wine, which we're not using, and removed compiler directive to try to get our environment variables to take precedence.
Added missing CMake invocations.
Use MXE for Travis cross-compile
Fix MXE dependencies
Abandon MXE in favour of Daniel Pfeifer's Travis CI cross-compilation setup.

MXE took too long for Travis CI.  Instead we are using a setup based on Daniel Pfeifer's example: https://github.com/purpleKarrot/CrossTest.
Fix toolchain file path.
Merge branch 'travis_setup'