private HttpResponse<JsonNode> jwtAuthPost(String assertion) throws UnirestException {
    HttpResponse<JsonNode> jsonResponse = Unirest.post(AUTH_URL)
              .header("accept", "application/json")
              .field("grant_type", JWT_GRANT_TYPE)
              .field("client_id", this.clientId)
              .field("client_secret", this.clientSecret)
              .field("assertion", assertion)
              .asJson();  //error here

    return jsonResponse;
}

Exception in thread "main" java.lang.NoClassDefFoundError: org/apache/http/nio/conn/NHttpClientConnectionManager
  at com.mashape.unirest.http.HttpClientHelper.prepareRequest(HttpClientHelper.java:154)
  at com.mashape.unirest.http.HttpClientHelper.request(HttpClientHelper.java:134)
  at com.mashape.unirest.request.BaseRequest.asJson(BaseRequest.java:68)