修改办法：在project facet中修改(因为不能直接修改，所以采取了这个折中的办法)
I had similar troubles in eclipse and the only way to fix it for me was to
- Remove the web module
- Apply
- Change the module version
- Add the module
- Configure (Further configuration available link at the bottom of the dialog)
- Apply
转自： https://stackoverflow.com/questions/19661135/dynamic-web-module-3-0-3-1/19663190#19663190