configuration save locking (jspm#63)

lib/config.js

@@ -315,7 +315,13 @@ function hasProperties(obj) {
   return false;
 }
 
+
+var savePromise;
 exports.save = function() {
+  // if already saving, wait for current save to complete before saving again
+  if (savePromise)
+    return savePromise.then(exports.save);
+
   var hasConfig = config.doCreate || false;
 
   var curConfig = config.curConfig = config.curConfig || {};
@@ -375,7 +381,7 @@ exports.save = function() {
 
   curConfig.bundles = config.bundles;
 
-  return Promise.resolve()
+  return savePromise = Promise.resolve()
   .then(function() {
     if (!hasConfig || config.configFile)
       return;
@@ -512,6 +518,9 @@ exports.save = function() {
     // save package.json
     return asp(fs.writeFile)(path.resolve(config.dir, 'package.json'), JSON.stringify(pjson, null, 2));
   })
+  .then(function() {
+    savePromise = undefined;
+  });
 }
 
 // convert NodeJS or Bower dependencies into jspm-compatible dependencies