Updated coveralls gem to 0.5.7 to fix Travis crash

Hi all, the pertinent update to the coveralls-ruby gem was adding a ":utf-8" here:

```ruby
File.open(file.filename, "rb:utf-8").read
```

Before it was crashing during the Travis build with this error:

```
Encoding::UndefinedConversionError
956"\xC3" from ASCII-8BIT to UTF-8
```

As seen on this job:
https://travis-ci.org/wycats/thor/jobs/5078933

Thanks!