Use non-deprecated bind/bind.hpp header

[vaboca]

boost/bind.hpp is deprecated and recommends using boost/bind/bind.hpp instead, see: https://github.com/boostorg/bind/blob/develop/include/boost/bind.hpp

[chwarr]

Can you add a brief changelog entry to the Unreleased section, as this looks like it will fix build warnings that some people are seeing?

Overall, the change LGTM. Which Boost version first added boost::placeholders? That information will need to be included in the final commit message. (I can write it, when I have the info.)

[chwarr]

It looks like the AppVeyor CI build is broken due to a change in the image names that AppVeyor uses. It doesn't look like this code is the cause of the break.

However, I cannot merge this change until that is fixed.
I've opened issue #1038 about this.

[chwarr]

We got a fix for #1038. I've merged that into this PR's branch.

[chwarr]

I've added the changelog line. This is ready for merging after the CI build finishes.