clippy: use is_empty() over len() == 0

```
src/config.rs:209:12: 209:39 warning: length comparison to zero, #[warn(len_zero)] on by default
src/config.rs:209         if config_instances.len() == 0 {
                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~
src/config.rs:209:12: 209:39 help: consider using `is_empty`
src/config.rs:            if config_instances.is_empty() {
src/config.rs:209:12: 209:39 help: for further information visit https://github.com/Manishearth/rust-clippy/wiki#len_zero
```

Signed-off-by: Paul Osborne <[email protected]>

Author: posborne

src/config.rs

@@ -206,7 +206,7 @@ impl GpioConfig {
             config_instances.push(try!(Self::from_file(fragment)));
         }
 
-        if config_instances.len() == 0 {
+        if config_instances.is_empty() {
             Err(Error::NoConfigFound)
         } else {
             let mut cfg = config_instances.remove(0);