Fix #564, students unable to submit 0 mark in the mixevaL

When select 0 mark, system shows require field are not completed. When
select a mark other than 0, the evaluation can be submitted, however,
when changing the mark to 0 and after saving, it changed back to the
original value.

The LOM should start with 1. Zeros are all for comments questions.
The LOM values are inconsistent between evaluations with and without
zero marks. E.g. with zero marks, LOM starts with 0, and without zero
marks, LOM starts with 1.
Update requirement to PHP 5.3
Update guard plugin
Fix cron email job failed when register_argc_argv is off

In case of register_argc_argv value is turned off in the php.ini.
Sending email job will fail with message: CakePHP Console: This file has
been loaded incorrectly and cannot continue. Turning it back on
from command line.
Fix #567, grade release button failed when install in subdir
Bump version to 3.1.2
Merge branch '3.1.x'