fix: ignore invalid package.json during read-pkg-up

[bcoe]

fixes #485

@avribacki I'm leaving in the read-pkg-up functionality, because it's an important part of being able to configure yargs (it's how you toggle on and off different parser settings). Having said this:

• I've consolidated the code into one method that caches the package once it's read, and is more performant (fix: lazy-load package.json and cache. get rid of pkg-conf dependency. #544).
• I've fixed the crasher reported in Yargs crashes when an invalid package.json is the directory tree. #485.

[coveralls]

Coverage remained the same at 100.0% when pulling 497194d on 485-fix into 2609b2e on master.