Bumping Atmosphere to latest version, 2.0.5

Current Scalatra Atmosphere support in 2.3.x is Atmo 2.0.3. 

Current Atmosphere version is 2.0.5

Issues fixed for 2.0.4 and 2.0.5 are at:

https://github.com/Atmosphere/atmosphere/issues?labels=2.0.4&milestone=&page=1&state=closed
https://github.com/Atmosphere/atmosphere/issues?labels=2.0.5&milestone=&page=1&state=closed