Raise an exception if RbNaCl has already been loaded

If this happens, it means a systemwide libsodium is avalable and has
already been loaded.

Fix finding libsodium.so with ruby 2.5

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>

Add RuboCop

Move project to the crypto-rb GitHub organization

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