fix(cache): use lodash assign for better node 0.12 support

Previously we used Object.assign, but for better node 0.12 support we can use _.assign. This also
fixes a small wording change in some tests.

Fixes #132

Author: jimthedev

src/commitizen/cache.js

@@ -24,7 +24,7 @@ function setCacheValueSync(cachePath, key, value) {
   } catch (e) {
     originalCache = {};
   }
-  var newCache = Object.assign(originalCache, {
+  var newCache = _.assign(originalCache, {
     [key]: value
   });
   fs.writeFileSync(cachePath, JSON.stringify(newCache, null, '  '));

test/tests/staging.js

@@ -24,7 +24,7 @@ beforeEach(function() {
   repo.createEndUser(sh, config.paths.endUserRepo);
 });
 
-describe('commit', function() {
+describe('staging', function() {
 
   it('should determine if a repo is clean', function(done) {