Add license element to project root pom.xml

license file in project root directory:

There is already a LICENSE.txt in project root directory:
https://github.com/jberet/jsr352/blob/master/LICENSE.txt

license element in project root pom.xml:

However, we also need to add <license> element to project root pom.xml.

An example of such license element:

<licenses>
    <license>
  	<name>GNU General Public License v3.0 only</name>
  	<url>http://www.gnu.org/licenses/gpl.txt</url>
    </license>
</licenses>

Use the URL from the appropriate entry as the <url> entry:
http://repository.jboss.org/licenses/

Use the name from spdx as the <name>:
https://spdx.org/licenses/

license file packaged into jar artifacts:

• in JBeret project master branch (1.3.0 or later), a LICENSE.txt is already packaged into jar:

jar tvf ~/.m2/repository/org/jberet/jberet-core/1.3.0.Beta6/jberet-core-1.3.0.Beta6.jar | grep -i license
 11251 Fri Jun 09 15:21:04 EDT 2017 META-INF/LICENSE.txt

• in JBeret project 1.2.x branch, no license file in jar artifacts.

jboss-batch-api_spec project (https://github.com/jboss/jboss-batch-api_spec)

already contains a LICENSE.txt, includes the LICENSE.txt in artifact jar file, and <license> element in pom.xml.

Made the minor change for the license name (changing Apache License to Apache License 2.0).