Check if version is defined instead of exists

Seems that `exists` still recognizes the key as existing but the
variable isn't necessarily `defined`.

Solves for errors related to:
```
Use of uninitialized value $version in concatenation (.) or string at /usr/bin/mk-build-deps line 548.
```

Signed-off-by: Eli Uriegas <eli.uriegas@docker.com>