build: Intl: deps: bump ICU to 56.1 (GA)

• ICU 56 was just released yesterday. Update to it.
• Notable changes: Unicode 8, CLDR 28, 2-3x number format perf, 20% improvement in Collator startup
  • more at http://site.icu-project.org/download/56 or in #2917 (closed)

Fixes: https://github.com/nodejs/node/issues/2917

@jasnell if you can land this in master also it would be great otherwise I will do later