icu - security update

It was discovered that the International Components for Unicode (ICU)
library mishandles converter names starting with x-, which allows
remote attackers to cause a denial of service (read of uninitialized
memory) or possibly have unspecified other impact via a crafted file.

For the stable distribution (jessie), this problem has been fixed in
version 52.1-8+deb8u3.

For the testing distribution (stretch), this problem has been fixed
in version 55.1-5.

For the unstable distribution (sid), this problem has been fixed in
version 55.1-5.

We recommend that you upgrade your icu packages.