travis: install mirage-types from here, check everything still builds

Previously we were installing the 1.1.0 release version of mirage-types
and building other repos against that. It's probably better to install
this version of mirage-types to detect pull requests which make incompat
changes.

Signed-off-by: David Scott <dave.scott@citrix.com>
travis: build './main.native'

The tests in lib_test need this.

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #249 from djs55/travis-latest

travis: install mirage-types from here, check everything still builds