update xerrors to latest for go1.13 compability
switch to modules (requirement for above)
remove phantomjs stub (replaced with grafana-image-renderer)
cleanup conf.d file
go recently received a security upgrade to 1.12.8 (f4894bf9) this
upgrade fixes various CVEs in go standard libraries, for instance in
HTTP/2 functionality. Since go is statically linked packages using this
functionality need to be rebuild.