Update to juttle 0.3.1.

This fixes this problem:

[2016-01-20 17:58:43.560] [ERROR] demo-job-manager - child-process-error Unhandled rejection TypeError: serializer.serialize is not a function
    at PluggableJSON._serialize (/Users/mstemm/work/src/outrigger/node_modules/juttle-jsdp/node_modules/pluggable-json/lib/index.js:64:35)
    at PluggableJSON.serialize (/Users/mstemm/work/src/outrigger/node_modules/juttle-jsdp/node_modules/pluggable-json/lib/index.js:53:41)
    at Object.serialize (/Users/mstemm/work/src/outrigger/node_modules/juttle-jsdp/lib/protocol/index.js:21:30)
    at send (/Users/mstemm/work/src/outrigger/lib/juttle-subprocess.js:61:23)
    at /Users/mstemm/work/src/outrigger/lib/juttle-subprocess.js:177:13

And was related to this:

montagejs/collections#139

package.json

@@ -41,7 +41,7 @@
     "immutable": "^3.7.5",
     "isomorphic-fetch": "^2.2.0",
     "jquery": "^2.1.4",
-    "juttle": "^0.3.0",
+    "juttle": "^0.3.1",
     "juttle-client-library": "^0.2.0-rc.1",
     "juttle-elastic-adapter": "^0.2.0",
     "juttle-gmail-adapter": "^0.4.0",