Fixing deprecation warning on HttpClient begin

[salqadri]

Was getting the following warnings:
/Users/foo/Documents/Arduino/libraries/firebase-arduino-master/src/FirebaseHttpClient_Esp8266.cpp: In member function 'virtual void FirebaseHttpClientEsp8266::begin(const string&, const string&)':
/Users/foo/Documents/Arduino/libraries/firebase-arduino-master/src/FirebaseHttpClient_Esp8266.cpp:31:86: warning: 'bool HTTPClient::begin(String, uint16_t, String, bool, String)' is deprecated (declared at /Users/foo/Library/Arduino15/packages/esp8266/hardware/esp8266/2.3.0/libraries/ESP8266HTTPClient/src/ESP8266HTTPClient.h:141) [-Wdeprecated-declarations]
http_.begin(host.c_str(), kFirebasePort, path.c_str(), true, kFirebaseFingerprint);

Followed the recommendation at https://github.com/esp8266/Arduino/blob/master/libraries/ESP8266HTTPClient/src/ESP8266HTTPClient.h:
bool begin(String host, uint16_t port, String uri, String httpsFingerprint); // deprecated, use the overload above instead bool begin(String host, uint16_t port, String uri, bool https, String httpsFingerprint) __attribute__ ((deprecated));

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[salqadri]

I signed it!

[proppy]

Fixes #143