RFC: amap was deprecated, using mapslices instead

Looks fine, comparison with R (using Rif):

```
julia> cor_spearman([1.5,1.5,2.2,2,3.5,3.55] , [1,2,3,4,5,6])
0.9276336570439175

julia> R("cor(c(1.5,1.5,2.2,2,3.5,3.55),c(1,2,3,4,5,6),method=\"spearman\")")[1]
0.9276336570439175
```
Merge pull request #10 from diegozea/patch-1

RFC: amap was deprecated, using mapslices instead