Update edd-ajax.js

Purchase button in checkout needed a more targeted jquery specifier to change its text back from Please wait... to Purchase.
Before, if there was an error in the purchase form, the Apply button (for discount codes) and any Login button would also change their text to 'Purchase'! They still worked fine, but the text was wrong - up to three buttons on the page all said 'Purchase' at that point.
Simplified jQuery accessor
Add check for mysqli_get_client_version
Switch to esc_attr
Merge pull request #2373 from ghost1227/issue/2369

Add check for mysqli_get_client_version
Merge pull request #2374 from ghost1227/issue/2372

Switch to esc_attr
Merge pull request #2370 from danlester/master

Update edd-ajax.js