revert changes to .gitignore

Author: lucywang000

.gitignore

@@ -56,7 +56,9 @@ marko-*/build
 marko-*/src/xtable.marko
 **/hyperoop/.rpt2_cache
 
+# keep root yarn.lock in repo so that travis will install yarn
 yarn.lock
+!/yarn.lock
 
 package-lock.json