Use the new json function in INGInious templates

[GuillaumeDerval]

This line will fail if there is a </script> tag into the json:
https://github.com/Maxmawt/INGInious-problems-blockly/blob/master/inginious-problems-blockly/templates/box_blockly.html#L10

Let's fix it using the new json() function introduced by INGInious/INGInious#337
(wait until the PR is merged, of course ;-))

[GuillaumeDerval]

PR has been merged on UCL-INGI/INGInious :-)