Findopm-parser.cmake: make sure that the ERT libraries are available

and some other minor cleanups to the module. For me it still works,
but this being the build system, I can't guarantee more...
Findopm-parser.cmake: invoke FindBoost unconditionally

even if boost was found previously, it might still miss a few
libraries. thanks to @akva2 for catching this.
Merge pull request #599 from andlaus/add_ERT_to_opm-parser

explicitly add the ERT libraries to the prerequisites of opm-parser