Log Message:
Update go-viper to 1.3.1:

1.3.1:

Make the map in MergeConfigMap case insensitive

1.3.0:

Restrict Travis to >= Go 1.11, use Go Modules, and get the test to pass

1.1.0:

* Added a new `DecoderConfigOption` type allowing the user to write custom
  functions that can override the default mapstructure.DecoderConfig
  settings

* Added a new `DecodeHook` function which returns
  a `DecoderConfigOption`. This allows the user to easily set their own
  Decode hooks when Unmarshaling

* Updated Unmarshal, UnmarshalKey and defaultDecoderConfig to support variadic
  trailing `DecoderConfigOption` functions to allow for customisation of
  the default  mapstructure.DecoderConfig

* Added a test case with example usage