Constraint core.109.07.00 as Linux-only due to various incompatibilities, notably on MacOS X.

This should prevent forced upgrades by mistake that will always break on this
version.  Note that you require the latest opam
(3fb1a97d72bd00bae05122c844b48b28704e87af) to have a solver that can handle
this situation.
Merge pull request #364 from mirage/master

constraint core.109.07.00 as Linux-only