use older third party revision

ref mcuadros/homebrew-hhvm#85

I've found that problem and possible solution.

CMake function HHVM_INSTALL_HEADERS does not exists in HHVM-3.1.0 branch, so we have to revert hhvm-third-party libs to an older revision e7bc03023a1b0311cabdb4272894453cc188e866.