Polish
Be defensive about solr classes required

Update SolrAutoConfiguration to specifically require HttpSolrServer
and CloudSolrServer classes rather than the generic SolrServer.

This prevents the rather cryptic "@ConditionalOnMissingBean annotations
must specify at least one bean" error that can occur if an earlier
version of solr happens to be on the classpath (as is the case when
hibernate-search is used).

Fixes gh-1098