fix e2e check test

The output of `kubectl version` has changed with newer kubectl version
from

```
{
  "serverVersion": ...,
  "clientVersion": ...
}
```

to

```
{
  "serverVersion": ...,
  "clientVersion": ...,
  "kustomizeVersion": ...
}
```

So the `kustomizeVersion` field is new which causes the JSON
unmarshaling to fail.

We now just unmarshal it to `map[string]interface{}` and peel the
server git version out of that map manually w/o unmarshalling the JSON
into a custom type.

Signed-off-by: Max Jonas Werner <mail@makk.es>