Fix yarn version bump for private packages (#257)

This allows np to create a tag on a private package, it sort of reverts
#235.
For public packages, the tag will be created using yarn publish.

index.js

@@ -98,6 +98,16 @@ module.exports = (input, opts) => {
 	}
 
 	tasks.add([
+		{
+			title: 'Bumping version using Yarn',
+			enabled: () => opts.yarn === true,
+			skip: () => {
+				if (runPublish && !pkg.private) {
+					return 'Public package: version will be bumped using yarn publish.';
+				}
+			},
+			task: () => exec('yarn', ['version', '--new-version', input])
+		},
 		{
 			title: 'Bumping version using npm',
 			enabled: () => opts.yarn === false,