Changed connection string from “sqlite” to “sqlite3” to please SociSQL

SociSQLDatabase.cpp:499 looks for the string "sqlite" not "sqlite3",  which results in a "libc++abi.dylib: terminating with uncaught exception of type soci::soci_error: Failed to find shared library for backend sqlite".

Correct backend name should be sqlite3. see http://soci.sourceforge.net/doc/3.2/backends/sqlite3.html