Fix gemspec

Hi,

I was having the following error while bundling:
```
curb at /Users/leo/.rbenv/versions/1.9.3-p327/lib/ruby/gems/1.9.1/bundler/gems/curb-0d11a1ff0212 did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
If you need to use this package without installing it from a gem repository, please contact todd.fisher@gmail.com and ask them to modify their .gemspec so it can work with `gem build`.
The validation message from Rubygems was:
  ["tests/bug_issue102.rb", "tests/foo.rb"] are not files
```

So I deleted these two (missing) files from the gemspec. I am unsure it was the right way to solve this, but it seems to work now.