Fix pull_licenses_java returning 404 from opensource.org

[bvolpato]

Apparently opensource.org changed the paths to reach license from https://opensource.org/licenses/mit-license.php -> https://opensource.org/licenses/mit/

This change fixes recent snapshot breakages.

For example: https://ci-beam.apache.org/job/beam_Publish_Beam_SDK_Snapshots/4836/console

11:17:12 ERROR:root:Invalid url for pcollections-2.1.2: http://www.opensource.org/licenses/mit-license.php. Retrying...
11:17:12 Traceback (most recent call last):
11:17:12   File "/home/jenkins/jenkins-slave/workspace/beam_Publish_Beam_SDK_Snapshots/src/sdks/java/container/license_scripts/pull_licenses_java.py", line 58, in pull_from_url
11:17:12     url_read = urlopen(Request(url, headers={

[chamikaramj]

LGTM. Thanks.

[chamikaramj]

Will merge after tests.