Demo: Fix captcha and marketo demo

Captcha demo did not link to the correct folder for jquery-validate.js, and also had an unnecessary .htaccess allowing execution of jpg files as php (which is needless since there are no jpg in that folder). This is a very minor security risk, but probably not one that needs to be kept around.

Fixed content type of image being incorrectly reported as text/html, which caused browser warnings.

Marketo demo also had a broken link to jquery-validate.js