README.md: use more recent compiler versions in build examples
README.md: use more recent compiler versions in build examples

roystgnr/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,roystgnr/libmesh,jwpeterson/libmesh,libMesh/libmesh,roystgnr/libmesh,dschwen/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,libMesh/libmesh,jwpeterson/libmesh,dschwen/libmesh,roystgnr/libmesh,roystgnr/libmesh,jwpeterson/libmesh,roystgnr/libmesh,dschwen/libmesh,dschwen/libmesh,libMesh/libmesh,libMesh/libmesh,BalticPinguin/libmesh,libMesh/libmesh,libMesh/libmesh,dschwen/libmesh,dschwen/libmesh,libMesh/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,libMesh/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,dschwen/libmesh,dschwen/libmesh,BalticPinguin/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh