.travis.yml: Switch to latest Agda version

Somehow, Agda does not install any more on Travis:
https://travis-ci.org/inc-lc/ilc-agda/builds/32258151
.travis.yml: Also install up-to-date happy

See
https://travis-ci.org/inc-lc/ilc-agda/builds/32259400
.travis.yml: Also install up-to-date alex

This exhausts the build-tools from Agda.cabal:

https://github.com/agda/agda/blob/2.4.0.2/Agda.cabal

Hopefully this will be fully sufficient, finally.
Add build status image to README.md

I noticed after looking for the "build: failing" image on the README.
(The build failure is stupid and is being fixed).
Draft conversion to CBPV types