Update lodash to 3.7, since we depend on _.get.
Update lodash to 3.7, since we depend on _.get.

`_.get` is used [in the library](https://github.com/namshi/winston-graylog2/blob/master/lib/winston-graylog2.js#L70), but that method is [only available from lodash v3.7 and higher](https://github.com/lodash/lodash/wiki/Changelog#v370).
namshi/winston-graylog2,myrlund/winston-graylog2