bitcoin: fix boost dependency

Bitcoin 0.15.1 doesn't build with boost 1.66.
I'm hesitant to apply untested patches to software like Bitcoin.
Instead I'm forcing the boost dependency to version 1.64 (which is the version listed @
https://github.com/bitcoin/bitcoin/blob/45173fa6fca9537abb0a0554f731d14b9f89c456/doc/dependencies.md).

Nothing in applications/altcoins/default.nix was using the boost162 parameter, so I've replaced it with the boost164 parameter.