TST: Fix assert_almost_equal error message
Most tests (in pandas' test suite and in general) are of form
`assert_almost_equal(result, expected)`.  Verbose error message was
treating its first argument as expected, this is now fixed.

Previous message was:

``` python
err_msg = lambda a, b: 'expected %.5f but got %.5f' % (a, b)
```

But a, the first argument, is actually `actual`, not expected.

Everywhere else, it makes sense to display `"%s != %s" % (a,b)`

can you hook up travis? (I know this is a trivial PR, but for the future)
 I set it up...hadn't realized I needed to enable it in travis-ci. Do you want me to redo this commit to trigger it?
 sure
git commit --amend -C HEAD
will force a Travis update
 got it - thanks!
 @wesm this looks fine for 0.11
