fix(publish): Checkout manifest changes serially

Dog-fooding is already paying dividends! This happened at the tail end of the v3.0.0-beta.0 publish:

```
lerna info published lerna
lerna ERR! Error: Command failed: git checkout -- core/*/package.json
lerna ERR! fatal: Unable to create '/Users/daniels/github/lerna/.git/index.lock': File exists.
lerna ERR!
lerna ERR! Another git process seems to be running in this repository, e.g.
lerna ERR! an editor opened by 'git commit'. Please make sure all processes
lerna ERR! are terminated then try again. If it still fails, a git process
lerna ERR! may have crashed in this repository earlier:
lerna ERR! remove the file manually to continue.
```

Running them serially instead of concurrently should avoid this race condition.

Author: evocateur

commands/publish/index.js

@@ -184,7 +184,7 @@ class PublishCommand extends Command {
 
     // reset since the package.json files are changed (by gitHead if not --canary)
     chain = chain.then(() =>
-      pMap(this.repository.packageConfigs, pkgGlob =>
+      pReduce(this.repository.packageConfigs, (_, pkgGlob) =>
         GitUtilities.checkoutChanges(`${pkgGlob}/package.json`, this.execOpts)
       )
     );