(bug 43801) add a getter for ICU version to ICUCollation

It will be necessary to be able to use correct version of Unicode
data files.

The constant INTL_ICU_VERSION this getter returns isn't really
documented. It is available since PHP 5.3.7 (see PHP bug 54561),
the getter will fail gracefully on older PHPs. It should be possible to
determine the ICU version on these by grepping the output of phpinfo(),
but I don't think such a minor improvement is worth such a huge hack.

Change-Id: Iee4b8380406ae71c980dfdd7b9fdd0b58ecb9cd0
Merge "(bug 43801) add a getter for ICU version to ICUCollation"