const JEDITOR = require( 'edit-json-file' );
const THEME_VERSION = new Date( ).toJSON( ).slice( 0, 10 ).replace( /-/g, '' );
const VERSION_FILE = JEDITOR( 'package.version.json' );

GULP.task( 'scripts-min', function ( ) {
...
VERSION_FILE.set( 'theme_js_version', THEME_VERSION );
VERSION_FILE.save( );
...
} );