travis: use container based infrastructure
travis: use container based infrastructure

This has various benifits. Most notably, builds start considerably
faster and perform slightly better.

cfelton/myhdl,josyb/myhdl,myhdl/myhdl,j-marjanovic/myhdl,jmgc/myhdl-numeric,cfelton/myhdl,j-marjanovic/myhdl,hgomersall/myhdl,j-marjanovic/myhdl,myhdl/myhdl,jmgc/myhdl-numeric,myhdl/myhdl,hgomersall/myhdl,jck/myhdl,juhasch/myhdl,gw0/myhdl,jck/myhdl,jandecaluwe/myhdl,juhasch/myhdl,gw0/myhdl,hgomersall/myhdl,josyb/myhdl,jck/myhdl,jandecaluwe/myhdl,josyb/myhdl,jandecaluwe/myhdl,jmgc/myhdl-numeric,gw0/myhdl,cfelton/myhdl,juhasch/myhdl