Don't list ignored Gemfile.lock in the gemspec.

`Gemfile.lock` is no longer version controlled, but remains in the gem specification. This prevents bundler from installing the binary if you reference git.

```ruby
# Gemfile
gem "clockwork", github: "tomykaira/clockwork"
```

```
clockwork at /Users/jgorset/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/bundler/gems/clockwork-c057ec2ae51e did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
The validation message from Rubygems was:
  ["Gemfile.lock"] are not files
```