Fix JS error preventing people from starring talks at LPW2015

[mfontani]

Reverts commit 0f91e321a04b3262657ca63dcfdf74e61bc82a4f

Revert "apparently, one must check whether a checkbox is checked or not using .prop() and no longer with .attr(), since jQuery 1.6+"

The commit was right, but act uses jQuery 1.3.2 which doesn't support prop() and therefore attr() is correct to be used.

[mfontani]

Please see if this can be had a look at since it impacts LPW2015.

i.e. go to http://act.yapc.eu/lpw2015/talks See console:

Uncaught TypeError: $(...).prop is not a function