Make a few fields final

Found via error-prone 2.3.1.

src/main/java/org/gaul/s3proxy/junit/S3ProxyRule.java

@@ -52,9 +52,9 @@ public final class S3ProxyRule extends ExternalResource {
     private final String endpointFormat;
     private final S3Proxy s3Proxy;
 
-    private BlobStoreContext blobStoreContext;
+    private final BlobStoreContext blobStoreContext;
     private URI endpointUri;
-    private File blobStoreLocation;
+    private final File blobStoreLocation;
 
     public static final class Builder {
         private AuthenticationType authType = AuthenticationType.NONE;

src/test/java/org/gaul/s3proxy/TestUtils.java

@@ -111,7 +111,7 @@ final class TestUtils {
 
     static final class S3ProxyLaunchInfo {
         private S3Proxy s3Proxy;
-        private Properties properties = new Properties();
+        private final Properties properties = new Properties();
         private String s3Identity;
         private String s3Credential;
         private BlobStore blobStore;