Release version 0.6.0 of gutenberg. (WordPress#2067)

Update package version to 0.5.0. (WordPress#1906)

Update plugin version to 0.4.0. (WordPress#1789)

Update plugin to version 0.3.0. (WordPress#1601)

Update plugin version to 0.2.0. (WordPress#1398)

Revert "Plugin: use beta with YYMMDD date format." (WordPress#1207)