Convert average_run_time to an integer. (#272)

There are two reasons for this change:

1. Having the average run time as a float gives the impression of being exact, whereas the actual run time wildly varies due to a wide variety of reasons (e.g. how busy it is on the server). That fractional component will almost never actually conform the real situation.

2. jq is often used to work with track config.json config files (e.g. to add elements to it), and it will remove any trailing .0 fractional part from a number, which caused configlet lint to fail. Those JQ scripts then have to work around this by manually adding .0 to it.

config.json

@@ -16,7 +16,7 @@
     "highlightjs_language": "haskell"
   },
   "test_runner": {
-    "average_run_time": 3.0
+    "average_run_time": 3
   },
   "files": {
     "solution": [