Add ruby 2.1 to the list of cross build ruby versions for Windows.

@luislavena released the RubyInstaller 2.1.3 for Windows some days ago, which is [not yet covered by the binary ffi gem](https://github.com/ffi/ffi/issues/368), but requires installation as source gem. Fortunately cross build runs fine for all 4 target x86 and 2 x64 versions in my rake-compiler-dev-box.

Unfortunately current rake-compiler requires a 3-part version number (2.1.1), so I set it to the [version that is currently used for the rake-compiler-dev-box](https://github.com/tjschuck/rake-compiler-dev-box/blob/master/bin/prepare_xrubies#L39) .

@tduehr What do you think - is the current state of the master branch ready to do a release? I'm waiting for the stdcall fixes to be released, too. :yum: