Lift ICU requirements

Hi! ext/intl presently requires ICU ≥ 4.0 (ICU 4.0 has been released on 2008-07-02[1]). Since ICU is still under vivid development our code is full of version checks (grep for ICU_VERSION), very old ICU versions are unlikely to be still around widespreadly, and we're generally not doing us a favor in supporting such old ICU versions (many PHPTs have multiple variants for different ICU versions). Therefore I have submitted PR #3487[2] which would raise the requirements to ICU ≥ 4.6 (ICU 4.6 has been released on 2010-12-02[3]). Anatol suggested to consider to lift the requirements even to ICU ≥ 50 (ICU 50.1 has been released on 2012-11-05[4]), or maybe even to ICU ≥ 52 (ICU 52.1 has been released on 2013-10-09[5]). What do you think? Does this require an RFC? [1] <http://icu-project.org/download/4.0.html#ICU4C> [2] <https://github.com/php/php-src/pull/3487> [3] <http://site.icu-project.org/download/46#ICU4C-Download> [4] <http://site.icu-project.org/download/50#TOC-ICU4C-Download> [5] <http://site.icu-project.org/download/52#TOC-ICU4C-Download> -- Christoph M. Becker