Remove use of deprecated OkHttp methods

[paulboocock]

{
	"resource": "/snowplow-java-tracker/src/main/java/com/snowplowanalytics/snowplow/tracker/http/OkHttpClientAdapter.java",
	"message": "The method create(MediaType, String) from the type RequestBody is deprecated",
	"startLineNumber": 131,
}

{
	"resource": "/snowplow-java-tracker/src/test/java/com/snowplowanalytics/snowplow/tracker/http/HttpClientAdapterTest.java",
	"message": "The method getUtf8Body() from the type RecordedRequest is deprecated",
	"startLineNumber": 118,
}