Workaround mangled javadocs (java 8+)

With Java 8, the build fails with the message `Execution failed for task ':javadoc_com.alkacon.opencms.calendar'`. This patch ignores javadoc errors during the build for Java 8+.

Log follows (warnings skipped):
````
[...]/alkacon-oamp/com.alkacon.opencms.calendar/src/com/alkacon/opencms/calendar/CmsSerialDateSelectWidget.java:64: error: unknown tag: layout
 * <code><layout element="Change" widget="SerialDateSelectWidget" configuration="property:calendar.startdate|count:25" /></code>.<p>
         ^

[...]
:javadoc_com.alkacon.opencms.calendar
1 error
22 warnings
:javadoc_com.alkacon.opencms.calendar FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':javadoc_com.alkacon.opencms.calendar'.
````