Usage example in README

[rcnlee]

Hello,

Does the usage example in the readme still work? It seems that the Environment function signature doesn't exist?

julia> function foo(n)
       x=n+1
       ((BigInt[1 1; 1 0])^x)[2,1]
       end
foo (generic function with 1 method)

julia> interp=enter(foo, Environment(Dict(:n=>20), Dict{Symbol,Any}()))
ERROR: MethodError: no method matching ASTInterpreter.Environment(::Dict{Symbol,Int64}, ::Dict{Symbol,Any})
Closest candidates are:
  ASTInterpreter.Environment(::Any, ::Any, ::Any, ::Any) at C:\Users\rcnlee\.julia\v0.5\ASTInterpreter\src\ASTInterpreter.jl:47
  ASTInterpreter.Environment{T}(::Any) at sysimg.jl:53

Thanks!