Image files are cached and not updated #99

Hi,
today I encountered a strange bug while using the new VipsImage(File) constructor. When an image is loaded once, then the file is changed and then the image is loaded again using the file constructor, it seems that not the data from the file is loaded, but that libvips uses some cache which is not invalidated.

Steps to reproduce:

1. Read an image using the new VipsImage(File) constructor
2. Update the image file (I checked afterwards that the file was really changed)
3. Read the image again using the file constructor
4. Save the read image somewhere else
   => Then the saved image is equal to the old image.