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:
rodjek/rspec-puppet#111
bundler/bundler#2287
Merge pull request #58 from eLobato/master

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