fix(IDN): use INTL_IDNA_VARIANT_UTS46 for idn_* functions

[layershifter]

This PR:

• adds PHP 7.2 to test matrix
• as INTL_IDNA_VARIANT_2003 is deprecated in PHP 7.2, we use INTL_IDNA_VARIANT_UTS46 for idn_* functions if it's defined

[codecov-io]

Codecov Report

Merging #20 into master will decrease coverage by 1.11%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master      #20      +/-   ##
============================================
- Coverage       100%   98.88%   -1.12%     
- Complexity       70       72       +2     
============================================
  Files             4        4              
  Lines           175      179       +4     
============================================
+ Hits            175      177       +2     
- Misses            0        2       +2

Impacted Files	Coverage Δ	Complexity Δ
src/IDN.php	87.5% <100%> (-12.5%)	8 <0> (+2)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c27974a...5a72923. Read the comment docs.