Exclude source jars from jar lookup

Added negative lookbehind assertion to ensure that the source jar is **not** picked up if it is present.

For example, with this structure the old picked up the `hadoop-streaming-2.2.0-test-sources.jar` jar, with the fixed version the correct `hadoop-streaming-2.2.0.jar`.

```
$ ls -gG */*streaming*.jar
-rw-rwxr-x 1 102785 Jan 11 21:47 lib/hadoop-streaming-2.2.0.jar
-rw-rwxr-x 1  71832 Jan 11 21:47 sources/hadoop-streaming-2.2.0-sources.jar
-rw-rwxr-x 1  76358 Jan 11 21:47 sources/hadoop-streaming-2.2.0-test-sources.jar
```
Merge pull request #861 from santtu/patch-2

Exclude source jars from jar lookup