Fixed omnibus.sh shebang line, now uses /bin/bash

Otherwise fails with error "curl: not found"

```
sh /etc/chef/bootstrap.sh 11.6.2
#    /etc/chef/bootstrap.sh: 10: [: ==: unexpected operator
#    /etc/chef/bootstrap.sh: 14: /etc/chef/bootstrap.sh: curl: not found

bash /etc/chef/bootstrap.sh 11.6.2
#    Using chef-solo 11.6.2 at /usr/bin/chef-solo
```