* 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