Exclude spec files from gem package

[amatsuda]

Here's a patch that excludes files under spec directory from the gem package.

With this patch, the gem package size shrinks as follows:

before: 60416 bytes
after:  47104 bytes

[geemus]

Thanks, I had done similar things on some of my gems and didn't realize I'd missed this one. I just did the same for fog-xml as well. Just let me know if you see any others and we'll get them fixed up also.

[amatsuda]

@geemus Thanks! I'll do so!