node_modules not getting cached

I was having a hard time getting node_modules cached and found that `updateCache` was never getting called from the `installPackages` callback:

```
installPackages(context, context.cachier('modules'), context.dataDir, config.caching, config.update_cache, function (err, exact) {
    if (err || exact || nocache) return next(err)
    updateCache(context, context.cachier('modules'), context.dataDir, next)
})
```
I logged the value of `exact` and found out it to be the output of the `npm install`.  `exact` is set `true` when the `package.json` hashes match

Changing 
`if (err || exact || nocache) return next(err)` 
to 
`if (err || exact === true || nocache) return next(err)` 
fixed it for me.