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

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/solr/SolrAutoConfiguration.java

@@ -37,7 +37,7 @@ import org.springframework.util.StringUtils;
  * @since 1.1.0
  */
 @Configuration
-@ConditionalOnClass(SolrServer.class)
+@ConditionalOnClass({ HttpSolrServer.class, CloudSolrServer.class })
 @EnableConfigurationProperties(SolrProperties.class)
 public class SolrAutoConfiguration {