Clean up dependencies

This commit was created by manually removing github.com/ugorji/go/codec from
the go.mod (which now does not exist - see ugorji/go#299)
and running `go mod tidy`.

Closes spf13#658

Remove superflous insensitiviseMaps in Unmarshal methods

Fixes spf13#482

Make the map in MergeConfigMap case insensitive

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

Fix nil pointer on watch function (spf13#568)

Fix nil pointer on watch function (spf13#568)

Fix nil pointer on watch function (spf13#568)

Release

Support customising mapstructure.DecoderConfig for Unmarshal

* 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

Release