Tweak gemspec
Tweak gemspec

We got rid of README.txt in b772049. Instead of just updating that ref,
replace the dodgy blacklist-based code for selecting files with a
whitelist based on `git ls-files`.

wincent/command-t,yrro/command-t,wincent/command-t,yrro/command-t,mtimkovich/command-t,wincent/command-t,mtimkovich/command-t