[packages] transmission: update to 2.81

git-svn-id: svn://svn.openwrt.org/openwrt@37468 3c298f89-4303-0410-b956-a3cf2f4a3e73
linux: remove 3.7 support

It is not used by any target.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt@37469 3c298f89-4303-0410-b956-a3cf2f4a3e73