PHP 7 Compatibility

Hi there, thanks for plugin.

I was checking compatibility for PHP7.

Following warnings found.
For your information.

350 | WARNING | Method name “SimpleEmailService::__triggerError” is discouraged; PHP has reserved all method names with a double underscore prefix for future use
532 | WARNING | Method name “SimpleEmailServiceRequest::__responseWriteCallback” is discouraged; PHP has reserved all method names with a double underscore prefix for future use
546 | WARNING | Method name “SimpleEmailServiceRequest::__customUrlEncode” is discouraged; PHP has reserved all method names with a double underscore prefix for future use
557 | WARNING | Method name “SimpleEmailServiceRequest::__getSignature” is discouraged; PHP has reserved all method names with a double underscore prefix for future use
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