Currently, there are build configuration files for CI, `travis.rb` and
`appveyor.rb`, but they are used for GCC/Clang and MSVC, not for Travis CI
and Appveyor, respectively.
Therefore, rename them to `gcc-clang.rb` and `msvc.rb`, respectively, and
move them under `build_config/ci/` to clarify that they are for CI.
KOBAYASHI Shuji