Use plist.parse instead of plist.parseFileSync.

When running Appium, I got a warning / error stating that:

    `parseFileSync()` is deprecated. Use `parseStringSync()` instead.

and my task was halted.

Appium uses `"plist": "~1.0.0"` in `package.json`.

`plist` v1.0.0 and up says that `parseFileSync()` is deprecated, and
that it should be replaced with `parseStringSync()`.  However,
`parseStringSync()` is *also* deprecated, to be replaced with `parse()`.
This all is based on:

 - https://github.com/TooTallNate/plist.js/blob/v1.0.0/lib/node.js#L15
 - https://github.com/TooTallNate/plist.js/blob/v1.0.0/lib/parse.js#L16

So I changed the related line in the iOS configuration file to point to
the new method `parse()`. This appears to resolve this error and allows
me to continue.
Use file contents as input to plist function.

plist `parse()` wants XML string, not a filename as the parameter. I
also changed the parameter to `parseXmlPlistFile()` to more clearly
indicate that it is receiving a filename, not a file or file contents.
Merge pull request #4181 from panozzaj/master

Use plist.parse instead of plist.parseFileSync