Sanity upgrade fails on unexpected output

[bjoerge]

If a sanity studio has a postinstall script that outputs something to stdout, sanity upgrade will fail with a JSON parse error. I suspect this may also happen if any package in the dependency tree has postinstall script that outputs something else than valid json, but haven't been able to confirm it.

Steps to reproduce:

1. Run sanity init
2. Manually install a previous version of any of the packages, e.g with npm install @sanity/base@0.117.0
3. open package.json, and add the following line to the scripts-section:

   "postinstall": "echo 'postinstall'"
   

4. Run sanity upgrade

I get the error:

undefined:1
postinstall
^

SyntaxError: Unexpected token p in JSON at position 0
    at DestroyableTransform.parse [as mapper] (<anonymous>)
    at DestroyableTransform.transform [as _transform] (~/.nvm/versions/node/v8.8.0/lib/node_modules/@sanity/cli/node_modules/split2/index.js:33:21)
    at DestroyableTransform.Transform._read (~/.nvm/versions/node/v8.8.0/lib/node_modules/@sanity/cli/node_modules/readable-stream/lib/_stream_transform.js:182:10)
    at DestroyableTransform.Transform._write (~/.nvm/versions/node/v8.8.0/lib/node_modules/@sanity/cli/node_modules/readable-stream/lib/_stream_transform.js:170:83)
    at doWrite (~/.nvm/versions/node/v8.8.0/lib/node_modules/@sanity/cli/node_modules/readable-stream/lib/_stream_writable.js:406:64)
    at writeOrBuffer (~/.nvm/versions/node/v8.8.0/lib/node_modules/@sanity/cli/node_modules/readable-stream/lib/_stream_writable.js:395:5)
    at DestroyableTransform.Writable.write (~/.nvm/versions/node/v8.8.0/lib/node_modules/@sanity/cli/node_modules/readable-stream/lib/_stream_writable.js:322:11)
    at Socket.ondata (_stream_readable.js:639:20)
    at emitOne (events.js:120:20)
    at Socket.emit (events.js:210:7)

[rexxars]

Fixed in #331