Travis: install PETSc 3.4 from PPA
install.sh: Preserve environment when running as user
install.sh: Install PETSc 3.4 from PPA
install.sh: create log file in PYOP2_DIR if it exists
Add a note about the amcg/petsc3.4 PPA to the README
Merge pull request #282 from OP2/petsc-ppa

Install PETSc binary package from amcg/petsc3.4 PPA