Ignore/warn about by file: dependency but do not fail

[drjasonharrison]

For various development reasons I am having to make changes to two packages under development so I have changed my packages.json dependency for package A's depedency on package B from a semver to a file:
"package_b": "file:../package_b",

Unfortunately next-update gets a bit confused by this type of dependency and should probably skip it. Instead, it looks into the package_a node_modules directory for package_b/package.json and fails to find it.

cannot find file /Users/.....package_a/node_modules/package_b/package.json
ERROR: cannot find module ndg-rest-server
ERROR testing next working updates
Error: Current installation is incomplete. Please run `npm install` or `bower install` first
    at /usr/local/lib/node_modules/next-update/src/next-update.js:51:26
    at _combinedTickCallback (internal/process/next_tick.js:73:7)
    at process._tickCallback (internal/process/next_tick.js:104:9)
From previous event:
    at checkCurrentInstall (/usr/local/lib/node_modules/next-update/src/next-update.js:69:10)
    at Object.<anonymous> (/usr/local/lib/node_modules/next-update/bin/next-update.js:64:3)
    at run (bootstrap_node.js:423:7)
    at startup (bootstrap_node.js:147:9)
    at bootstrap_node.js:538:3