single \ to properly escape . in check_has_ipv4_address

```
Failure/Error: it { should have_ipv4_address('10.1.1.129') }
[#]        expected #has_ipv4_address?("10.1.1.129") to return true, got false
[#]        /bin/sh -c ip\ addr\ show\ tun0\ \|\ grep\ \'inet\ 10\\.1\\.1\\.129/\'
```

```
root@localhost:~# ip addr show tun0 | grep 'inet 10\\.1\\.1\\.129'
root@localhost:~#
```
```
root@localhost:~# ip addr show tun0 | grep 'inet 10\.1\.1\.129'
    inet 10.1.1.129 peer 10.1.1.130/32 scope global tun0
root@localhost:~#
```