Fix a bug that "gem install" use "ruby" platform rather than specific platform

"gem install" with "--platform" option should install the specified
platform gem rather than "ruby" platform.

For example, "gem install --platform=x86-mingw32 gtk2" should install
"gtk2-X.Y.Z-x86-mingw32" not "gtk2-X.Y.Z".

How to confirm the problem:

    % sudo gem install --platform=x86-mingw32 gtk2 --verbose
    ...
    Downloading gem gtk2-2.2.3.gem
    GET https://api.rubygems.org/gems/gtk2-2.2.3.gem

The command should download "gtk2-2.2.3-x86-mingw32.gem".
Merge pull request #1093 from kou/install-platform-specific-gem

Fix a bug that "gem install" use "ruby" platform rather than specific platform