Boost was only used for program option parsing. To avoid this huge
dependency, this commit replaces boost::program_options with cxxopts,
a header-only library with the same functionality.
cxxopts is added as a submodule, and Boost is removed from the
dependencies in the code and Travis configuration.
As Travis only provides outdated packages (compilers in particular),
this changes the Travis configuration to use Docker images to build and
test the code. This also has the benefit that multiple distributions can
be tested and not just Ubuntu.
For the time being, Arch Linux and Ubuntu 18.04 are added as supported
platforms.
