This should make crane able to build images without any cache (if specified)

Hi, 

This is the pull request for #35

First time writing Go so I had a little problem with
```go
if nocache { 
something 
} else {
somethingelse
}
```

Thanks!

Signed-off-by: Scott M. Likens <scott@likens.us>
Can you move the executeCommand out of the `if/else` block? That would be a little cleaner. Also, you could use `append` like so:
```
args := []string{"build"}
if nocache {
    args = append(args, "--no-cache")
}
// append the rest and execute command
```