Bind grn_column_truncate() that is exported since Groonga 4.0.9
Remove unneeded setup in TruncateTest
Bump up required Groonga version

4.0.8 ->
4.0.9
doc: Fix document for truncate column
Merge pull request #41 from cosmo0920/bind-column-truncate

Bind grn_column_truncate() that is exported since Groonga 4.0.9

Patch by Hiroshi Hatake. Thanks!!!