Allow RandomNonceBox to decrypt the same cipher text multiple times

https://github.com/cryptosphere/rbnacl/edit/master/lib/rbnacl/random_nonce_box.rb#L105 was preventing RandomNonceBox to open the same enciphered message multiple times
Add Ruby 2.1.0, remove rbx-19
Merge pull request #97 from Asmod4n/patch-1

Allow RandomNonceBox to decrypt the same cipher text multiple times