apply latest capture format

Hi. I was trying to avoid the warning/error on new capture syntax at elixir 0.10.3 and master. 
I couldn't pass the "Unsupported expression in pipeline" error, but making the test pass with & -> fn(x) temporary changes (is there any easy fix for pipeline part?). 

** (ArgumentError) Unsupported expression in pipeline |> operator: Enum.map(&&1 * 2)
    lib/amrita/elixir/pipeline.ex:86: Amrita.Elixir.Pipeline.pipeline_error/1
    lib/amrita/elixir/pipeline.ex:54: Amrita.Elixir.Pipeline.pipeline_op/2
    lib/amrita/elixir/pipeline.ex:11: Amrita.Elixir.Pipeline.pipeline_op/2
    test/unit/amrita/elixir/t_pipeline.exs:119: Amrita.Elixir.Pipeline.|>/2