be more precise about elpa package dir names

ELPA Package directories are named <package>-<version>.  Previously
`el-get-elpa-package-directory' looked for package-dirs using
`all-completions' meaning anything starting with <package> would match.
This is problematic if a package is a prefix of another package, eg the
`s' and `solarized-theme' packages.

Instead of doing that, look for a directory that is precisely
<package>-<version> where <version> is something acceptable to
`version-to-list'.

Fixes #1189.