Only index latest charm version.

Use elasticsearch versioning to make sure that only
the latest version of the charm or bundle is indexed.
Therefore only returning the latest version in search
results.
Merge pull request #178 from mhilton/search-index-only-latest-version

Only index latest charm version.

Use elasticsearch versioning to make sure that only
the latest version of the charm or bundle is indexed.
Therefore only returning the latest version in search
results.