Refresh repository contents before attempting to install packages.

This commit resolves #1. I've tested it by removing
.emacs.d/elpa/* (using -rfv flags on rm, which clobbers the package
contents cache file) and then running
package-installer-install-dependencies, which appears to work in such a
situation now.

Credit for bringing this attention to my priority goes to @samercier.
Merge branch 'fix-package-contents-not-refreshing' into 'emacs'

Signed-off-by: Kristofer Rye <kristofer.rye@gmail.com>