travis: drop linux ppa testing which over-complicates multi-os support
travis: drop linux ppa testing which over-complicates multi-os support

fengmk2/node-pre-gyp,synthetos/node-pre-gyp,hybridgroup/node-pre-gyp,fengmk2/node-pre-gyp,mapbox/node-pre-gyp,synthetos/node-pre-gyp,synthetos/node-pre-gyp,hybridgroup/node-pre-gyp,mapbox/node-pre-gyp,fengmk2/node-pre-gyp,fengmk2/node-pre-gyp,hybridgroup/node-pre-gyp,etiktin/node-pre-gyp,mapbox/node-pre-gyp,synthetos/node-pre-gyp,hybridgroup/node-pre-gyp,mojodna/node-pre-gyp,mojodna/node-pre-gyp,mapbox/node-pre-gyp,mojodna/node-pre-gyp,fengmk2/node-pre-gyp,synthetos/node-pre-gyp,mapbox/node-pre-gyp,mojodna/node-pre-gyp,etiktin/node-pre-gyp,etiktin/node-pre-gyp,mojodna/node-pre-gyp,etiktin/node-pre-gyp,etiktin/node-pre-gyp