Travis build configuration matrix

[joshua-white]

We currently have 9-10 build targets on Travis, and we should revisit our coverage strategy:

Options:

• Pay more for better Travis VMs
• Lighten the Travis testing, but add in LC Gitlab for extensive coverage (perhaps just prior to merge)
• Trim our current matrix (do we need both gcc8.2 and 8.3, etc)
• Not build LVArray tests when building GEOSX (strong consensus for this one)

[TotoGaz]

I think #1700 will conclude this discussion?

EDIT: Should we open an issue for the LvArray idea?

[paveltomin]

Probably not relevant anymore after #2571