granting binaries executable permissions

When I build the gem, I see warnings about the `bin/` scripts missing Unix executable permissions. This commit grants these permissions, clearing up the warning.

```
$ gem build shlint.gemspec
WARNING:  bin/shlint is not executable
WARNING:  bin/checkbashisms is not executable
  Successfully built RubyGem
  Name: shlint
  Version: 0.1.6
  File: shlint-0.1.6.gem
/Users/andrew/Desktop/src/shlint$ gem install shlint-0.1.6.gem 
Successfully installed shlint-0.1.6
Parsing documentation for shlint-0.1.6
Installing ri documentation for shlint-0.1.6
1 gem installed
```