attempt to fix method ambiguity on Julia v0.5

Author: jrevels

src/dual.jl

@@ -80,9 +80,9 @@ macro define_binary_dual_op(f, both_body, left_body, right_body)
             end
         end
 
-        @inline $(f){T,S,X,Y,N}(x::Dual{T,Dual{S,X,N}}, y::Dual{T,Dual{S,Y,N}}) = $both_body
-        @inline $(f){T,S,V,N}(x::Dual{T,Dual{S,V,N}}, y::Dual{S}) = $left_body
-        @inline $(f){T,S,V,N}(x::Dual{S}, y::Dual{T,Dual{S,V,N}}) = $right_body
+        @inline $(f){T,S,X<:Real,Y<:Real,N}(x::Dual{T,Dual{S,X,N}}, y::Dual{T,Dual{S,Y,N}}) = $both_body
+        @inline $(f){T,S,X<:Real,Y<:Real,N}(x::Dual{T,Dual{S,X,N}}, y::Dual{S,Y,N}) = $left_body
+        @inline $(f){T,S,X<:Real,Y<:Real,N}(x::Dual{S,X,N}, y::Dual{T,Dual{S,Y,N}}) = $right_body
     end)
 end