+mirage-types-lwt, which makes sure that mirage-types.lwt is available
use mirage-types-lwt instead of mirage-types
travis: switch to using opam 1.2
travis: speed up by using 2 jobs in parallel
make mirage depend explicitly on `crunch` since it shells out to it
Revert "travis: speed up by using 2 jobs in parallel"

This reverts commit fb0c44496d633af749f394f0614f0275b558cf97, as it
runs slower in Travis despite it having 1.5 vcpus
Merge pull request #30 from avsm/master

add mirage-types-lwt