Fixed libdwarf dist URI
Added jemalloc 3.0 as external dependency for HHVM
Updated HHVM dependencies

- Jemalloc and Google glog were moved to external dependencies
- Raised minimal required version of boost
- Fixed required version of libdwarf
- MySQL dev-db dependency replaced by virtual
- Added libvpx dependency
Added HHVM release 2.3.1