Fixed: test_convolve(Image2_UT) in Image2.rb fails #72

From ImageMagick 6.8.4, this error occurred.
Because some argument validation was deleted.

deleted validation:
```
  if ((width % 2) == 0)
    ThrowImageException(OptionError,"KernelWidthMustBeAnOddNumber");
```
<cite>[effect.c (ImageMagick 6.8.3)](https://github.com/trevor/ImageMagick/blob/master/branches/ImageMagick-6.8.3/magick/effect.c#L933)</cite>
<cite>[effect.c (ImageMagick 6.8.4)](https://github.com/trevor/ImageMagick/blob/master/branches/ImageMagick-6.8.4/magick/effect.c#L877)</cite>

Therefore, another exception should be raised.
I selected "memory allocation failed" (Allocation size=-1)

```
assert_raise(Magick::ImageMagickError) { @img.convolve(-1, [1.0, 1.0, 1.0, 1.0]) }
```