[Tests] unpin istanbul-api

 - istanbuljs/istanbuljs#216 (comment)

Author: ljharb

package.json

@@ -92,7 +92,7 @@
     "glob-gitignore": "^1.0.9",
     "in-publish": "^2.0.0",
     "istanbul": "^1.0.0-alpha.2",
-    "istanbul-api": "~1.2",
+    "istanbul-api": "^1.3.7",
     "json-loader": "^0.5.7",
     "karma": "^1.3.0",
     "karma-chrome-launcher": "^1.0.1",