Build Windows binary packages

We can cross compile Windows binary on docker using
rake-compiler-dock and rake-compiler.
`rake gem:windows` will pull docker image, cross compile and
create the following packages in `pkg` directory.
- rbnacl-libsodium-x.x.x-x64-mingw32.gem
- rbnacl-libsodium-x.x.x-x86-mingw32.gem

make compatible with 64 bit

update to recent versions in travis

drop support for ruby lower than 2.2.6

Support OpenBSD

OpenBSD builds libsodium.so.19.1 (for example), so the libsodium*.so won't match.

Apparently in some build environments (e.g. Windows) the "symlinked"
version is not always created. This should (hopefully) always find it.

Support for loading libsodium.dll on Windows

Fix issues for Windows MinGW

- configure fails because `sh` is not executed
- load fails due to no libsodium.so

This fix works with [RubyInstaller](http://rubyinstaller.org/).
Fix #6

Update libsodium 1.0.1