ignore dot files in the gemspec file list

The dot files .gemtest, .gitignore, and .travis.yml are automatically
included in the main "git ls-files" command, but we don't need to ship
these to the to users of the gem. It's better to exclude these dot files
from the gem packaging altogether.
Merge pull request #202 from ktdreyer/nodotfiles

ignore dot files in the gemspec file list