remove use of deprecated function

[proppy]

/home/proppy/Arduino/libraries/firebase-arduino/src/FirebaseHttpClient_Esp8266.cpp: In member function 'virtual void FirebaseHttpClientEsp8266::begin(const String&, const String&)':
/home/proppy/Arduino/libraries/firebase-arduino/src/FirebaseHttpClient_Esp8266.cpp:29:70: warning: 'bool HTTPClient::begin(String, uint16_t, String, bool, String)' is deprecated (declared at /home/proppy/arduino-1.6.8/hardware/esp8266com/esp8266/libraries/ESP8266HTTPClient/src/ESP8266HTTPClient.h:141) [-Wdeprecated-declarations]
     http_.begin(host, kFirebasePort, path, true, kFirebaseFingerprint);
                                                                      ^