hhvm minimal cmake version requirements

Should not matter actually, as Gentoo tree does not have such old cmake
versions anyway, but could be helpful, if somebody tries to install hhvm
via our ebuilds on some old system, not updated for some time.

https://github.com/facebook/hhvm/commit/3d242a6d9da457429a47dc9f33f5e9b3c629c4d0