Upgrade maven-assembly-plugin to 3.3.0 to fix file permissions

[ato]

The old default version of maven-assembly-plugin generates packages
containing dangerous world writable files.

Before:

$ zipinfo dist/target/heritrix-3.4.0-SNAPSHOT-dist.zip | grep heritrix[^/]*.jar
-rwsrwsrwt  2.0 unx   677008 b- defN 21-Jul-19 15:12 heritrix-3.4.0-SNAPSHOT/lib/heritrix-engine-3.4.0-SNAPSHOT.jar
-rwsrwsrwt  2.0 unx   446139 b- defN 21-Jul-19 15:12 heritrix-3.4.0-SNAPSHOT/lib/heritrix-modules-3.4.0-SNAPSHOT.jar
-rwsrwsrwt  2.0 unx   148876 b- defN 21-Jul-19 15:12 heritrix-3.4.0-SNAPSHOT/lib/heritrix-commons-3.4.0-SNAPSHOT.jar

After:

$ zipinfo dist/target/heritrix-3.4.0-SNAPSHOT-dist.zip | grep heritrix[^/]*.jar
-rw-r--r--  2.0 unx   677008 b- defN 21-Jul-19 15:23 heritrix-3.4.0-SNAPSHOT/lib/heritrix-engine-3.4.0-SNAPSHOT.jar
-rw-r--r--  2.0 unx   446139 b- defN 21-Jul-19 15:23 heritrix-3.4.0-SNAPSHOT/lib/heritrix-modules-3.4.0-SNAPSHOT.jar
-rw-r--r--  2.0 unx   148877 b- defN 21-Jul-19 15:23 heritrix-3.4.0-SNAPSHOT/lib/heritrix-commons-3.4.0-SNAPSHOT.jar

Fixes #413