doc: Remove outdated information about boost versions

Bitcoin core should work with any remotely recent boost version
if a proper build environment is present. Remove a confusing comment
from the build documentation.

Rebased-From: bd45b1a

Conflicts:
	doc/build-unix.md
build: Remove message about Ubuntu 13.10 when no boost sleep implementation found

It's only confusing people into thinking that they should mess with
boost versions, which should not be necessary to get bitcoind to work.

If there is a bug in the build system with autodetecting boost it needs
to be solved not worked around.

Rebased-From: 539abc4729ea16039d148cfa3b771929f7d37584
Added notes about Berkeley DB version incompatibility under Debian.
Added note about incomplete OpenSSL libraries on Fedora.
Added early-stage 1.8.1 release notes.
Corrected credit for AndyMeows in release notes.
Merge pull request #709 from rnicoll/1.8.1-docs

Correct out of date build documentation