Net::SCP should be a class

`bundle exec rake test` raises the following error
```
/.../net-scp/lib/net/scp/upload.rb:3:in `<module:Net>': SCP is not a class (TypeError)
/.../net-scp/lib/net/scp/version.rb:2: previous definition of SCP was here
```

Author: mishina2228

lib/net/scp/version.rb

@@ -1,5 +1,5 @@
 module Net
-  module SCP
+  class SCP
     # A class for describing the current version of a library. The version
     # consists of three parts: the +major+ number, the +minor+ number, and the
     # +tiny+ (or +patch+) number.