PETSc 3.5 fix for no-default-comm-world.

Refs #2348.

Thanks @milljm for finding this.
Merge pull request #3596 from jwpeterson/petsc_3.5

PETSc 3.5 fix for no-default-comm-world.
Merge commit '1d059818b8676f7924a5f5c490257e6ce1c0751c'