Manage Dependencies With Godep

As I see at [https://github.com/tools/godep#file-format](https://github.com/tools/godep#file-format),  
`GoVersion`  
should be  
`go1.3`  
Currently, Gin only support that.
(NOTE: if you want compatibility, please check [`app_engine` branch](https://github.com/gin-gonic/gin/tree/app_engine).)