parseChangelog always return null when getting "Version", "Distribution" and "Source"

HI,

I'm trying to use this plugin but it is always returning null for the values is getting at function parseChangelog, as it's seen in my log:

```
bash -c 'cd "/opt/jenkins/workspace/release-deb-test/web/debian" && dpkg-parsechangelog -lchangelog'
[debian-package-builder] Determined latest version to be null
...
+ dch --check-dirname-level 0 -b --distribution null --newVersion 2016.3.3+dev19 -- 'Build #19. Started by an SCM change.'
dch warning: Recognised distributions are:
{precise,trusty,wily,xenial}{,-updates,-security,-proposed,-backports} and UNRELEASED.
Using your request anyway.
dch: Did you see that warning? Press RETURN to continue...
...
ERROR: Build step failed with exception
java.lang.IllegalArgumentException: Null value not allowed as an environment variable: DEBIAN_SOURCE_PACKAGE
```

If dpkg-parsechangelog is executed over my changelog, I obtain the following:

```
Source: package
Version: 2016.3.3
Distribution: trusty
Urgency: low
Maintainer: SysAdmins
Date: Tue, 08 Mar 2016 09:03:49 +0100
Changes:
package (2016.3.3) trusty; urgency=low
.
* Initial debian realase.
```

I inspected parseChangelog function and it seems correct to me; the only thing makes me suspect is the function `runner.runCommandForOutput`.

Finally, my java version is the following:

```
ubuntu@build-1:/opt/jenkins/workspace$ java -version
java version "1.7.0_80"
Java(TM) SE Runtime Environment (build 1.7.0_80-b15)
Java HotSpot(TM) 64-Bit Server VM (build 24.80-b11, mixed mode)
```

Greetings,
Pablo