Fix Access Denied errors on Windows

```python
C:\dev\code\sandbox\conda_test> conda build .
An unexpected error has occurred, please consider sending the
following traceback to the conda GitHub issue tracker at:

    https://github.com/conda/conda-build/issues

Include the output of the command 'conda info' in your report.


Traceback (most recent call last):
  File "C:\dev\bin\Anaconda\Scripts\conda-build-script.py", line 4, in <module>
    sys.exit(main())
  File "C:\dev\bin\Anaconda\lib\site-packages\conda_build\main_build.py", line 111, in main
    args_func(args, p)
  File "C:\dev\bin\Anaconda\lib\site-packages\conda_build\main_build.py", line 309, in args_func
    args.func(args, p)
  File "C:\dev\bin\Anaconda\lib\site-packages\conda_build\main_build.py", line 269, in execute
    build.build(m, verbose=not args.quiet, post=post)
  File "C:\dev\bin\Anaconda\lib\site-packages\conda_build\build.py", line 287, in build
    rm_rf(config.short_build_prefix)
  File "C:\dev\bin\Anaconda\lib\site-packages\conda\install.py", line 162, in rm_rf
    shutil.rmtree(path)
  File "C:\dev\bin\Anaconda\lib\shutil.py", line 247, in rmtree
    rmtree(fullname, ignore_errors, onerror)
  File "C:\dev\bin\Anaconda\lib\shutil.py", line 247, in rmtree
    rmtree(fullname, ignore_errors, onerror)
  File "C:\dev\bin\Anaconda\lib\shutil.py", line 247, in rmtree
    rmtree(fullname, ignore_errors, onerror)
  File "C:\dev\bin\Anaconda\lib\shutil.py", line 247, in rmtree
    rmtree(fullname, ignore_errors, onerror)
  File "C:\dev\bin\Anaconda\lib\shutil.py", line 252, in rmtree
    onerror(os.remove, fullname, sys.exc_info())
  File "C:\dev\bin\Anaconda\lib\shutil.py", line 250, in rmtree
    os.remove(fullname)
WindowsError: [Error 5] Access is denied: u'C:\\dev\\bin\\Anaconda\\envs\\_build\\Lib\\site-packages\\numpy\\core\\libifcoremd.dll'
```