call package-initialize in elpa installion

The package-install function calls some initalization functions from
within the interactive form.  This can cause it to fail when called
non-interactively.

package-refresh-contents calls package-read-all-archive-contents so we
don't need to call it explicitly.

Fixes #1389
Merge branch 'package-initialize' of https://github.com/npostavs/el-get into npostavs-package-initialize
Merge branch 'npostavs-package-initialize' into next