add support for locations in elm.json

[rogozind]

We have to use a non-standard "location" field in elm.json. This is required to workaround a bug in ELM. It looks like this:

    },
    "locations": {
        "elm/virtual-dom": {
            "method": "git",
            "url": "git@github.com:jinjor/virtual-dom.git",
            "ref": "fix-extension-bug"
        }
    }
},

Unfortunately IntelliJ does not understand this and errors out trying to load that ELM.json. I have to cut this out, open the file and then restore the section.

Could we adjust the extension to ignore this non-standard element?