Use babel-preset-env (#738)

Transpiles node v4-compatible in prepublish, current node when running tests

.babelrc

@@ -1,7 +1,28 @@
 {
-  "presets": ["es2015"],
+  "presets": [
+    ["env", {
+      "targets": {
+        "node": 4
+      }
+    }]
+  ],
   "plugins": [
     "transform-decorators-legacy",
     "add-module-exports"
-  ]
+  ],
+  "env": {
+    "lerna-test": {
+      "presets": [
+        ["env", {
+          "targets": {
+            "node": "current"
+          }
+        }]
+      ],
+      "plugins": [
+        "transform-decorators-legacy",
+        "add-module-exports"
+      ]
+    }
+  }
 }

package.json

@@ -68,7 +68,7 @@
     "babel-jest": "^19.0.0",
     "babel-plugin-add-module-exports": "^0.2.1",
     "babel-plugin-transform-decorators-legacy": "^1.3.4",
-    "babel-preset-es2015": "^6.24.0",
+    "babel-preset-env": "^1.3.2",
     "eslint": "^3.19.0",
     "eslint-config-babel": "^6.0.0",
     "eslint-plugin-babel": "^4.1.1",