Update checkpoint.py (#299)

* Update checkpoint.py

if edit "{save_dir}/last_checkpoint" by vim, "\n" will auto append to the end of file. which will raise 

```
FileNotFoundError: [Errno 2] No such file or directory: 'save_dir/model_0060000.pth\n'
```

* Update checkpoint.py

if edit "{save_dir}/last_checkpoint" by vim, "\n" will auto append to the end of file. which will raise
```
FileNotFoundError: [Errno 2] No such file or directory: 'save_dir/model_0060000.pth\n'
```

maskrcnn_benchmark/utils/checkpoint.py

@@ -79,6 +79,7 @@ class Checkpointer(object):
         try:
             with open(save_file, "r") as f:
                 last_saved = f.read()
+                last_saved = last_saved.strip()
         except IOError:
             # if file doesn't exist, maybe because it has just been
             # deleted by a separate process