Fix example code in README considering changes of DataFrames

With DataFrames v0.5.4, sample codes makes following errors.
To avoid errors, I fix readme codes.

```
julia> iris = data("datasets", "iris")
ERROR: no method data(ASCIIString, ASCIIString)
```

```
julia> labels = iris["Species"].data
WARNING: indexing DataFrames with strings is deprecated; use symbols instead
 in getindex at /Users/mic/.julia/v0.3/DataFrames/src/deprecated.jl:120
ERROR: type PooledDataArray has no field data
```

```
julia> instances = transpose(matrix(iris[:, 2:5]))
ERROR: matrix not defined
```