Intermittent error in Grunt replace task

[brian-learningpool]

Some users have been reporting intermittent errors in the Grunt replace task, such as this one raised against the authoring project:
adaptlearning/adapt_authoring#1132

[brian-learningpool]

After investigation it was discovered that this could be replicated by having undefined or null properties in the config and course objects. (I was able to re-create it on a previous version of a course that used Trickle but didn't have the _completionAttribute defined on config.json).

As the replace is only really required right now by the xAPI and (potentially) Spoor extension, I'm proposing a fix to only allow replacement of a subset of the possible attributes.