Update README to check out SDK 1.13.2 instead of 1.12

Some breaking changes in recent Firefox builds have been fixed in SDK 1.13 (and the two point releases based off of it). Using SDK 1.12 against the current Firefox Nightly, you'd get a "tabs.open is undefined" error when you click the "Analyze" button. 

Switching to SDK 1.13.2 no longer throws that error.