Update to 1.21
Update DEPENDS

Upstream changes:
version 1.21: Mon  3 Jul 15:31:19 CEST 2017

Fixes:
- _prepend and _append texts doubled.
  rt.cpan.org#122304 [Andreas Koenig]
- accidental stringification of exceptions
  rt.cpan.org#122324 [Slaven Rezic]

Improvements:
- also \n of msgid moves to _prepend or _append

version 1.20: Tue 27 Jun 16:41:36 CEST 2017

Fixes:
- Dancer2 sporadic missing request. [Andrew Beverley]
- attribute _lang overrules default locale in translation
- remove Log::Report::Lexicon dependency from ::Translator
- formatter PRINTP cannot be used, remove docs which tell that
- exceptions triggered translation too often

Improvements:
- add textdomain($name, 'EXISTS')
- add textdomain($domain_object);
- short-cut when translating without context
- use String::Print::printi() to interpolate: that code was forked-off
  earlier, and now mature.

(wen)