fix ast slices

[dmamelin]

Issue #809 caught my attention, and here's what I found:

ast.Slice indeed has no value attribute. Maybe it existed at some point and changed in ast.py, but it's more likely a copy/paste mistake from the ast_index function right above.

In most cases this didn't surface because recurse_assign, ast_delete, and ast_subscript have special handling for ast.Slice.

Didn't add numpy tests - slices are already well covered.

[craigbarratt]

Nice catch & fix - thanks!