Enable bundler caching in travis
disable image button when there is no imageuploader
remove html_editor partial
Merge pull request #248 from wangqiang1208/bugfixing/disable-iamge-achievement-html-editor

Disable image button in achievement html editor
Merge pull request #244 from jsyeo/jsyeo-travis-cache

Enable bundler caching in travis
change can_start policy, if a student has ever started, let him start
refine code
Merge pull request #249 from wangqiang1208/features/mission-attempt-auth

if a student has ever started, let him start
Merge branch 'development' of github.com:Coursemology/coursemology.org into Deploy