Point to new master of program options.

The would include two commits:

https://github.com/boostorg/program_options/commit/0a7005d7c6b90fa3c6b10494920fcf4f499ec87f
https://github.com/boostorg/program_options/commit/9d7c9875265ec7fec03aabf18e3d53c3e5e1a46c

It's a pity that there's no way to examine commits that would be included automatically.