Update to JUnit 4.12

Currently JUnit Core is based on JUnit 4.11, in SLING-5352, some improvement could benefit from the enhancement implemented in JUnit 4.12 related to possibility to combine both ClassRule and Rule annotation for a static field.

That would be a great help to simplify the use case discussed in SLING-5352.

simply update the version on the pom.xml would be sufficient.

from 4.11

...
<properties>
<junit.version>4.11</junit.version>
<hamcrest.version>1.3</hamcrest.version>
<jacoco.version>0.6.2.201302030002</jacoco.version>
</properties>
...

to 4.12

...
<properties>
<junit.version>4.12</junit.version>
<hamcrest.version>1.3</hamcrest.version>
<jacoco.version>0.6.2.201302030002</jacoco.version>
</properties>
...