Bug #3893 - Avoid using git ls-files on gemspec

git ls-files does not always give the right result, and installations from source might fail to build the gem properly because of this (see http://pastie.org/8557757).

See the issues below for longer discussions on this:
https://github.com/rodjek/rspec-puppet/issues/111
https://github.com/bundler/bundler/issues/2287