icu: Upgrade icu to 50.1.2

We need a more recent version for Maui.
linux-maui-ostree: Don't add git sha1 to recipe version

Otherwise the path will be huge and make headers_install will fail because
the argument list is too long.

Linux 3.11 will have a patch that should fix this issue, see [1].
Also Yocto fixed it for their kernel headers package [2].

See also [3].

[1] = https://lkml.org/lkml/2013/6/6/525
[2] = http://git.yoctoproject.org/cgit.cgi/poky/commit/?id=53cc748b93e8af584557d6db5309c3e955182c5c
[3] = http://comments.gmane.org/gmane.linux.kbuild.devel/9455