boost: pass -dumpversion to user-config.jam

Short version avoids compiling problems with toolchains
containing long version string like Linaro toolchain.

[Peter: drop head/sed as -dumpversion returns version directly]
Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>