Merge pull request #6567 from dumganhar/v3

Adds download progress for download-deps.py  and adds some missing files
[AUTO][ci skip]: updating cocos2dx_files.json
Merge pull request #6568 from CocosRobot/update_cocosfiles_1399225034

[AUTO][ci skip]: updating cocos2dx_files.json
Updates download-deps.py, invalid content header check
Merge pull request #6569 from dumganhar/download-progress

Updates download-deps.py, invalid content header check
Removed external folder
Adds external submodule
Updates travis-script, removed using download-deps.py now, 'external' is
a submodule module now. please use `git submodule update --init` to
fetch it.