check against minimal needed version of ldc/dmd compilers
only the first three numbers are compared as numbers, everything after
that is string compared
* update configure after changes to configure.ac
* Update .travis-ci.sh
* Update Travis CI to download minimum version of DMD for x32 build testing
* Update Travis CI build script to use an LDC version string
* Add 'libxml2' to guest dependencies for AARM64
* Update ARMHF and AARM64 to use different versions of LDC due to due to https://github.com/ldc-developers/ldc/issues/3027
* Update to use US Debian mirror rather than ftp.debian.org due to slow network access. By using US mirror, 4 servers are available for download rather than just 1 when using ftp.debian.org