Disable libvips cache for unit tests

Aids memory leak detection

test/unit/blur.js

@@ -5,6 +5,8 @@ var assert = require('assert');
 var sharp = require('../../index');
 var fixtures = require('../fixtures');
 
+sharp.cache(0);
+
 describe('Blur', function() {
 
   it('specific radius', function(done) {