Change version of PETSc in the install script

PETSc 3.4 a la amcg is no longer sufficent. This changes the install
script to follow the currently required branches.
Update the manual install instructions
install.sh: remove libhdf5-openmpi-7 dependency
Update requirements.txt, requirements-minimal.txt
Update tox.ini
Travis: don't install PETSc from binary package
Merge pull request #337 from OP2/post_fluidity_install

Install instructions for a post-Fluidity world