Remove .gem file

the .gem file was causing issues when building the new gem.

> Gem::Package::TarWriter::FileOverflow: You tried to feed more data than fits in the file.
> An error occurred while installing resque-sliders (0.3.0), and Bundler cannot continue.
> Make sure that `gem install resque-sliders -v '0.3.0'` succeeds before bundling.

Seems to be fixed by removing the  .gem file:
https://github.com/rubygems/rubygems/issues/623