MSVC++ 1900+ always compile with C++11 mode enabled, so it should be
safe to selectively enable 'override'specifier for internal use.

It does not enable all C++11 features for SOCI and we still compile
with C++11 compilation mode SOCI_CXX_C11=OFF by default.

Refactoring performed with clang-tidy-4.0:

 cmake -DCMAKE_EXPORT_COMPILE_COMMANDS=ON -DSOCI_CXX_C11=ON ..
 run-clang-tidy.py -clang-tidy-binary -header-filter='.*' \
                   -checks='-*,modernize-use-modernize' -fix