Fix el-get-update-all for elpa packages

el-get-update-all overrode package-refresh-contents using flet to
avoid triggering a full refresh of all elpa packages when updating
each elpa package. The use of flet was incorrect, and the overridden
function was specified with different arguments, causing an error.
Merge pull request #1162 from rocha/master

Fix el-get-update-all for elpa packages