Upgrade to lerna 2.7 to correct peer dependency management (#1539)

lerna.json

@@ -1,5 +1,5 @@
 {
-  "lerna": "2.1.2",
+  "lerna": "2.7.1",
   "version": "independent",
   "npmClient": "yarn",
   "useWorkspaces": true

package.json

@@ -31,7 +31,7 @@
     "is-image": "^1.0.1",
     "is-url": "^1.2.2",
     "jsdom": "^11.5.1",
-    "lerna": "^2.1.2",
+    "lerna": "^2.7.1",
     "matcha": "^0.7.0",
     "mocha": "^2.5.3",
     "npm-run-all": "^4.1.1",