Fix el-get-elpa-update-available-p for Emacs >24.3
Call package-initialize in el-get-elpa-update

Otherwise, `package-alist` is nil when `el-get-elpa-update-available-p`
is called, which yield another "Wrong type argument: arrayp, nil" error.
Comment why checking available-package
Merge pull request #1639 from tkf/fix-el-get-elpa-update-available-p

Fix el-get-elpa-update-available-p for Emacs > 24.3