CHANGE: centralise configuration value for bootstrap version.

Once you get to this commit, you should delete any line in your base.jinja2 (or equivalent) that looks like this:

{% set trex_bootstrap_version = 3 %}

And instead, if your bootstrap version isn't 3, you will want to do this in default.ini:

[trex]
bootstrap_version = 2

In your javascript, you'll also want to add a script block like the following after the tag that includes trex.js:

<script>
Trex.opt.bootstrap_version = 2;
</script>

Neither change is necessary if you're using bootstrap 3, as 3 is the default.
Make flash_is work correctly for bootstrap 2 tests.