travis: Workaround for Boost <=1.56 being broken on Clang.
travis: Workaround for Boost <=1.56 being broken on Clang.

See https://svn.boost.org/trac/boost/ticket/7473.

blindsighttf2/Astron,blindsighttf2/Astron,blindsighttf2/Astron,blindsighttf2/Astron