fix part of issue #74 - find_last_of() finds any one character; replace with rfind()

For any string ending with S, the single-quote following the S was ignored. This caused
SCL to read beyond the end of the string and discard all remaining entities in the file.
std::string::find_last_of() finds any one character, rather than a specific sequence.
Replaced with std::string::rfind() for intended behavior.