Fixed failing test which assumes UTC time.

So I found an issue with the logic in the receiver test if which leads to test failures when using localtime.

```
% GOPATH=$HOME/Code/go go test l2met/receiver                                                                                                                                                                                                                              2013-06-29 17:00:08 markw ttys006
warning: building out-of-date packages:
	l2met/bucket
	l2met/store
installing these packages with 'go test -i l2met/receiver' will speed future tests.

at=receiver-drop b=2013-06-30 03:00:00 +1000 EST n=2013-06-29 17:00:00 +1000 EST
at=receiver-drop b=2013-06-30 03:00:00 +1000 EST n=2013-06-29 17:00:00 +1000 EST
at=receiver-drop b=2013-06-30 03:00:00 +1000 EST n=2013-06-29 17:00:00 +1000 EST
--- FAIL: TestReceiver (0.02 seconds)
	receiver_test.go:83: case=router actual-len=0 expected-len=3
FAIL
FAIL	l2met/receiver	0.054s
```

Somewhere in the code it is tainting the date without honouring the offset as illustrated in the pattern of the output.

To correct this I made sure the input to the test is UTC times which matches what I would expect in Heroku logs.

Note tests also pass if you pass UTC timezone when running the tests.

```
TZ=UTC GOPATH=$HOME/Code/go go test l2met/receiver
```

I tried to locate how the date in the bucket was getting tainted but I couldn't locate it, this fix mitigates it to some degree however it would be nice to locate the root cause.