Bugzilla Importer doesn't support Bugzilla 2.16.x

The docs state that JIRA supports importing from Bugzilla 2.16, 2.18 and 2.20. However, running it against v2.16.3 results in:

java.sql.SQLException: Column 'id' not found. 
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:910) 
at com.mysql.jdbc.ResultSet.findColumn(ResultSet.java:955) 
at com.mysql.jdbc.ResultSet.getInt(ResultSet.java:2570) 
at com.atlassian.jira.util.BugzillaImportBean.getAllBugzillaProjects(BugzillaImportBean.java:1642) 
...

The "id" column was introduced sometime after 2.16.3 in 2.18.

We need to amend the docs to correctly show which versions we do support, or fix the importer to be compatible with 2.16. Either way we should extend the bugzilla functional tests to include all supported versions - at the moment it only tests 2.20.