Undo changes to use gemspec to find executables

This is causing problems with omnibus caching. It will be easier to just specify the files we want to exclude explicitly.
cc @danielsdeleo 
We don't need to update this. AFICT, the actual problem with berks is that the gemspec does this:

```ruby
  s.files                     = `git ls-files`.split($\)
  s.executables               = s.files.grep(%r{^bin/}).map{ |f| File.basename(f) }
  s.test_files                = s.files.grep(%r{^(test|spec|features)/})
```

The problem occurs when we do a build using the git cache, because we have to remove the .git dir from the berks app in order for it to cache correctly. So we end up doing this:

```
irb(main):002:0> `git ls-files`.split($\)
fatal: Not a git repository (or any of the parent directories): .git
=> []
```

So if we need to adjust the gemspec in the tests to repro the issue, we should do:

```ruby
spec.executables = []
```