Resolve Gettext warnings about obsolete plural forms in PO files

[arturz]

On compilation of the application, there are bunch of warnings about obsolete Plural-Forms header.

Example:

warning: Plural-Forms headers in the form "nplurals=<int>" (without the "plural=<rule>" part following) are invalid and support for them will be removed in future Gettext versions. Make sure to use a complete Plural-Forms header, which also specifies the pluralization rules, or remove the Plural-Forms header completely. If you do the latter, Gettext will use its built-in pluralization rules for the languages it knows about (see Gettext.Plural).
For the "ru" locale, you can use the following header:

nplurals=3; plural=(n%10==1 && n%100 != 11 ? 0 : n%10>=2 && n%10<=4 && (n%100<10||n%100>=20) ? 1 : 2);

  (gettext 0.26.2) lib/gettext/plural.ex:282: Gettext.Plural.init/1
  (gettext 0.26.2) lib/gettext/compiler.ex:563: Gettext.Compiler.nplurals/3
  (gettext 0.26.2) lib/gettext/compiler.ex:509: Gettext.Compiler.compile_po_file/5
  (gettext 0.26.2) lib/gettext/compiler.ex:447: Gettext.Compiler.compile_unified_po_file/4
  (elixir 1.17.2) lib/enum.ex:1703: Enum."-map/2-lists^map/1-1-"/2
  (elixir 1.17.2) lib/enum.ex:1703: Enum."-map/2-lists^map/1-1-"/2

[jk-lamb]

I will do it.

[jk-lamb]

Done. Check the PR