Issue #49: Require tablelib.php

[petersistrom]

Resolves #49

Reproductsion/testing steps:

1. Spin up a clean MOODLE_401_STABLE
2. Install tool_encoded
3. Turn profiling off: php admin/cli/cfg.php --name=profilingenabled --set=0
4. Run scheduled task: php admin/cli/scheduled_task.php --execute="\tool_encoded\task\queue_base64_report"
   Should result in the error: Class 'flexible_table' not found
5. Apply patch, task from 4 should run successfully

[bwalkerl]

Thanks @petersistrom

I haven't been able to replicate this on an existing site, so I think it may also get loaded by something else. The fix looks good to me.