Use dig_rb instead of ruby_dig when backport_dig is not installed

[okkez]

Because ruby_dig 0.0.2 does not support a object that responds to dig
method properly. On the other hand, dig_rb 1.0.1 supports it properly.

[okkez]

For example, fluent-plugin-geoip uses geoip2_c's GeoIP2::LookupResult#dig with hash (pseudo code):

db = GeoIP2::Database.new(db_path)
result = db.lookup(ip)
keys = [:hostname, :city, :names, :en]
hash = { hostname: result }
hash.dig(*keys") # => "Mountain View"

If we use ruby_dig 0.0.2, the last line will return nil.