Bump required PETSc version

[Jan Blechta]

PETScDMCollection.cpp references to DMShellSetContext which seems to exist only in PETSc 3.7 while CMakeLists.txt requires now only 3.6.

~/dev/petsc(master)$ git tag --contains fef3a512d74581b4c9754d4f49732bc7c253d70a
v3.7
v3.7.1
v3.7.2
v3.7.3
v3.7.4

[Prof Garth Wells]

Merge branch 'garth/bump-petsc-version-3.7'. Fixes Issue #799.

→ <<cset 912834e7e56b>>