switch to virtual/opencl

Package-Manager: portage-2.2.8-r2
never version in gx86
drop old deps

Package-Manager: portage-2.2.8-r2
fix mvapich2 dep

Package-Manager: portage-2.2.8-r2
fixed geant-vmc deps

Package-Manager: portage-2.2.8-r2
fixed latex dep

Package-Manager: portage-2.2.8-r2
fixed latex dep

Package-Manager: portage-2.2.8-r2
.travis.yml: test different portage versions