Make Sensu::Socket support check results too large to fit in a single packet.

ATTN @portertech 
FYI @grepory
When `@data_buffer` contains kilobytes or even megabytes of data, the warning message can quickly fill the sensu-client log.  It would be good to truncate the output to avoid this.  Here's an alternative version: 

``` ruby
        @logger.warn('giving up on data buffer from client', {
          :data_buffer => truncate_text(@data_buffer),
          :last_parse_error => truncate_text(@last_parse_error.to_s),
        })
```

Here is the implementation of `truncate_text()`: 

```
    def truncate_text(text)
      if text.length > 1000
        text = "#{text[0..100]}...#{text[-100..-1]}"
      end

      text
    end
```

Both `@data_buffer` and `@last_parse_error.to_s` have to be truncated as `@last_parse_error.to_s` will sometimes contain a copy of the data buffer.  An example of this is the parse error: 

```
795: unexpected token at '{\"name\":\"cucumber-example.Failing.failing\",\"handlers\":[\"cucumber\"],\"output\"...AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA'
```