Error while Compling using <ESP8266HTTPClient.h> library

Using Arduino Programming Questions
1

The following example program was complied in Arduino 1.6.7 environment using <ESP8266HTTPClient.h> library

code:

#include <Arduino.h>
#include <ESP8266WiFi.h>
#include <ESP8266WiFiMulti.h>
#include <ESP8266HTTPClient.h>

#define USE_SERIAL Serial

ESP8266WiFiMulti WiFiMulti;

void setup() {

    USE_SERIAL.begin(115200);

    for(uint8_t t = 4; t > 0; t--) {
        USE_SERIAL.printf("[SETUP] WAIT %d...\n", t);
        USE_SERIAL.flush();
        delay(1000);
    }

    WiFiMulti.addAP("FiniteSpace", "password");

}

void loop() {
    // wait for WiFi connection
    if((WiFiMulti.run() == WL_CONNECTED)) {

        HTTPClient http;

          USE_SERIAL.print("[HTTP] begin...\n");
           http.begin("api.thingspeak.com", 80, "/talkbacks/7409/commands/execute?api_key=CXDELYYFS80S3AYR"); //HTTP

     
        int httpCode = http.GET();
        
        if(httpCode) {
            // HTTP header has been send and Server response header has been handled
            USE_SERIAL.printf("[HTTP] GET... code: %d\n", httpCode);

            // file found at server
            if(httpCode == 200) {
                String payload = http.getString();
                USE_SERIAL.println(payload);
            }
        } 
    }

    delay(1000);
}

error from the complier

/home/bharu/Arduino/libraries/ESP8266HTTPClient/src/ESP8266HTTPClient.cpp: In member function 'bool HTTPClient::beginInternal(String, const char*)':
/home/bharu/Arduino/libraries/ESP8266HTTPClient/src/ESP8266HTTPClient.cpp:151:32: error: 'base64' has not been declared
         _base64Authorization = base64::encode(auth);
                                ^
/home/bharu/Arduino/libraries/ESP8266HTTPClient/src/ESP8266HTTPClient.cpp: In member function 'void HTTPClient::setAuthorization(const char*, const char*)':
/home/bharu/Arduino/libraries/ESP8266HTTPClient/src/ESP8266HTTPClient.cpp:273:32: error: 'base64' has not been declared
         _base64Authorization = base64::encode(auth);
                                ^
/home/bharu/Arduino/libraries/ESP8266HTTPClient/src/ESP8266HTTPClient.cpp: In member function 'String HTTPClient::getString()':
/home/bharu/Arduino/libraries/ESP8266HTTPClient/src/ESP8266HTTPClient.cpp:640:5: error: 'StreamString' was not declared in this scope
     StreamString sstring;
     ^
/home/bharu/Arduino/libraries/ESP8266HTTPClient/src/ESP8266HTTPClient.cpp:640:18: error: expected ';' before 'sstring'
     StreamString sstring;
                  ^
/home/bharu/Arduino/libraries/ESP8266HTTPClient/src/ESP8266HTTPClient.cpp:644:13: error: 'sstring' was not declared in this scope
         if(!sstring.reserve((_size + 1))) {
             ^
/home/bharu/Arduino/libraries/ESP8266HTTPClient/src/ESP8266HTTPClient.cpp:650:20: error: 'sstring' was not declared in this scope
     writeToStream(&sstring);
                    ^
exit status 1
Error compiling.

I had installed base64 library, String stream library how to sort this problem