Follow opm-material phase name changes

Commit OPM/opm-material@30c90734ce renamed, among other things

    FluidSystem::gPhaseIdx -> FluidSystem::gasPhaseIdx
    FluidSystem::lPhaseIdx -> FluidSystem::liquidPhaseIdx

Adapt to that change.
Merge pull request #99 from bska/chase-phase-rename

Follow opm-material phase name changes