remove cassandra driver

we don't use it for Figure53.com's backend

fix cross compiling to linux/amd64

Fix cross-compiling to linux/amd64

by removing the sqlite driver, which we don't use

add go.mod, fix imports

Define explicit dependency versions via go 1.11 modules

and fix imports for `github.com/mattes/migrate`;
change to `github.com/Figure53/migrate`