.travis.yml: Split the 3 builds in 3 jobs

The 3 main targets of the Makefile are now built in 3 separate jobs.
This allow faster compilation, separate debugging and enforce there is
no interaction between the 3 targets.

Moreover, only wip/merge is built at the moment.

Signed-off-by: Sylvain Gault <sylvain.gault@gmail.com>