ledger: boost 1.56 compatibility

Patch from https://groups.google.com/forum/#!topic/ledger-cli/9HwEJcD0My4

Addresses build failure against boost 1.56 described at https://github.com/Homebrew/homebrew/pull/31428#issuecomment-56282997
No need to change this but you can use shorter URLs e.g. `https://github.com/ledger/ledger/commit/d5592e.diff`