fixed `packer --version` to show version again

As mentioned by @kensykora in #1753 I also first tried a `packer --version` after upgrading to Packer 0.7.5 and only got the usage, but no version number.

I also found the BATS script `test/cli.bats` which checks some packer options. This test script shows errors:

```bash
$ test/cli.bats 
 ✓ cli: packer should show help
 ✗ cli: packer version
   (in test file test/cli.bats, line 24)
     `[ "$status" -eq 1 ]' failed
   Executing:  packer version
   Packer v0.7.5
   Executing:  packer -v
   usage: packer [--version] [--help] <command> [<args>]
   Available commands are:
       build       build image(s) from template
       fix         fixes templates from old versions of packer
       inspect     see components of a template
       push        push template files to a Packer build service
       validate    check that a template is valid
       version     Prints the Packer version
   Executing:  packer --version
   usage: packer [--version] [--help] <command> [<args>]
   Available commands are:
       build       build image(s) from template
       fix         fixes templates from old versions of packer
       inspect     see components of a template
       push        push template files to a Packer build service
       validate    check that a template is valid
       version     Prints the Packer version
 ✓ cli: packer version show help
```

In the `main.go` I only added the `Version` field and now packer shows at least the version number:

```bash
$ packer version
Packer v0.7.5

$ packer --version
0.7.5

$ packer -v
0.7.5

$ echo $?
1
```

I have updated the `test/cli.bats` script as well to make the test work.