update min julia version to 0.6.0-pre

[tkelman]

this goes with JuliaDiff/ForwardDiff.jl#233

[coveralls]

Coverage remained the same at 58.974% when pulling ac5f38a on tk/minjuliaver into 1df5505 on master.

[codecov-io]

Codecov Report

Merging #10 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #10   +/-   ##
=======================================
  Coverage   58.97%   58.97%           
=======================================
  Files           2        2           
  Lines          39       39           
=======================================
  Hits           23       23           
  Misses         16       16

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