Reduce number of reftest chunks for master runs

Reduce number of reftest chunks to 5 to save some resources; these
never limit overall e2e time.

Author: jgraham

.taskcluster.yml

@@ -40,16 +40,11 @@ tasks:
               - [testharness, 13, 15]
               - [testharness, 14, 15]
               - [testharness, 15, 15]
-              - [reftest, 1, 10]
-              - [reftest, 2, 10]
-              - [reftest, 3, 10]
-              - [reftest, 4, 10]
-              - [reftest, 5, 10]
-              - [reftest, 6, 10]
-              - [reftest, 7, 10]
-              - [reftest, 8, 10]
-              - [reftest, 9, 10]
-              - [reftest, 10, 10]
+              - [reftest, 1, 5]
+              - [reftest, 2, 5]
+              - [reftest, 3, 5]
+              - [reftest, 4, 5]
+              - [reftest, 5, 5]
               - [wdspec, 1, 1]
             each(chunk):
               taskId: {$eval: 'as_slugid(browser.name + browser.channel + chunk[0] + str(chunk[1]))'}