Fix number of parallel repair computation

Downgrade to Dropwizard 1.0.7 and Guava 19.0 to fix dependency issues
Make repair manager schedule cycle configurable (was 30s hardcoded)

ref: #108

src/main/java/com/spotify/reaper/storage/CassandraStorage.java

@@ -8,6 +8,9 @@
 import java.util.List;
 import java.util.Set;
 import java.util.UUID;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.TimeUnit;
 import java.util.stream.Collectors;
 
 import org.slf4j.Logger;