Fix rake create_gem which fails by copying files to file not a directory

rake create_gem is currently broken as the copyOutputFiles treats 'bin' as a file name instead of a directory.  Therefore multiple files are copied and override a file bin.

By creating the directories first the behavour is as expected.

Error message on windows is

```  ripple.Testing -> C:\Projects\ripple\src\ripple.Testing\bin\Debug\ripple.Testing.dll
Cleaning directory bin
cp src/ripple/bin/Debug/FubuCore.dll bin
cp src/ripple/bin/Debug/NuGet.Core.dll bin
cp src/ripple/bin/Debug/ripple.exe bin
ON THE FLY SPEC FILES
bin/ripple
==========
rake aborted!
undefined method `build' for Gem::Package:Module
C:/Projects/ripple/rakefile.rb:69:in `block in <top (required)>'
Tasks: TOP => create_gem
(See full trace by running task with --trace)
```