Add PETSc to requirements-git.txt (#2065)

* Add PETSc to requirements-ext.git

This fixes an install issue where this was removed from PyOP2 in
OP2/PyOP2#617.

* Add petsc4py to packages list

* Move petsc4py location in script

In order for PyOP2 to install it needs to be installed AFTER petsc4py.

requirements-git.txt

@@ -4,3 +4,4 @@ git+https://github.com/FInAT/FInAT.git#egg=finat
 git+https://github.com/firedrakeproject/tsfc.git#egg=tsfc
 git+https://github.com/OP2/PyOP2.git#egg=pyop2
 git+https://github.com/dolfin-adjoint/pyadjoint.git#egg=pyadjoint
+git+https://github.com/firedrakeproject/petsc.git@firedrake#egg=petsc

scripts/firedrake-install

@@ -1568,7 +1568,7 @@ if mode == "install":
         run_pip(["install", "lazy-object-proxy==1.4.*"])
 
     packages = clone_dependencies("firedrake")
-    packages = clone_dependencies("PyOP2") + packages
+    packages = clone_dependencies("PyOP2") + ["petsc4py"] + packages
     packages += ["firedrake"]
 
     for p in options["packages"]: