attempt to fix a weird error on a jenkins test rig

http://opm-project.org/CDash/viewBuildError.php?type=0&buildid=21247

this *seems* to be caused by an old boost version, but I'm not
sure. It can't be the compiler because for me it works on GCC 4.4, GCC
4.9 as well as clang 3.3.

It would be nice if somebody could check that this brings the build server is back to normal before this PR is merged...