install.sh: pip install petsc == 3.3.7, petsc4py >= 3.4
install.sh: Trick the petsc4py installer to keep PETSc 3.3

petsc4py 3.4 depends on PETSc >= 3.4 (as per distutils requires), but
is known to work with PETSc 3.3-p7, which is still compatible with
Fluidity.