Check executable files strictly

Check executable files strictly when installing a Gem from RubyGems.

### Example

Basic usage:

```sh
$ gem specification bundler executables
---
- bundle
- bundler
```

Convert to shell friendly from a YAML format:

```sh
$ gem specification bundler executables | tail -n +2 | tr -d - | tr -d '\n'
 bundle bundler
```

### Refs

- [Specification Reference - RubyGems Guides](http://guides.rubygems.org/specification-reference/)