Upgrade boost 1.53 -> 1.55 to fix UA build on Ubuntu 14.04

Building boost succeeds on Ubuntu 12.04 but fails in the atomic module
on Ubuntu 14.04. Seems to be this bug (fixed in boost 1.55) related to a
change in glibc:

  https://svn.boost.org/trac/boost/ticket/8973
  https://svn.boost.org/trac/boost/changeset/84950