migrate to "export default"

also, set no-use-before-define in linter

src/df.js

@@ -7,11 +7,6 @@ import shellModule from './shell.js';
 const debug = debugModule('box:df');
 const shell = shellModule('df');
 
-export {
-    filesystems,
-    file,
-    prettyBytes
-};
 
 // binary units (non SI) 1024 based
 function prettyBytes(bytes) {
@@ -66,3 +61,9 @@ async function file(filename) {
     const lines = output.trim().split('\n').slice(1); // discard header
     return parseLine(lines[0]);
 }
+
+export default {
+    filesystems,
+    file,
+    prettyBytes
+};