[travis] use prebuilt verilator binary
[travis] use prebuilt verilator binary

Glyfina-Fernando/pymtl,Glyfina-Fernando/pymtl,cornell-brg/pymtl,cornell-brg/pymtl,jjffryan/pymtl,tj93/pymtl,cornell-brg/pymtl,Glyfina-Fernando/pymtl,jck/pymtl,tj93/pymtl,cfelton/pymtl,cfelton/pymtl,12yujim/pymtl,jck/pymtl,12yujim/pymtl,jck/pymtl,jjffryan/pymtl,12yujim/pymtl,tj93/pymtl,jjffryan/pymtl,cfelton/pymtl