Fix Julia 1.0 compatibility

In Julia 1.0, `I` is UniformScaling{Bool}, whereas (at least) Julia 1.3 and
later have added `I(n)` as shorthand for `Diagonal{Bool,Array{Bool,1}}`.

If you encounter the error:
    LoadError: MethodError: objects of type UniformScaling{Bool} are not callable
this is a one possible reason for that.

Worked around by defining a custom `eye` function that creates a `Diagonal`
populated with ones.

test/test_utilities.jl

@@ -14,8 +14,9 @@ using Test, Tensors, LinearAlgebra
 # Various tensors
 let Z3 = zeros(3, 3),
     O3 = ones(3, 3),
-    I3 = I(3)
-    @test isapprox(tovoigt(II()), I(6))
+    eye(n) = Diagonal([1 for _ in range(1, length=n)]),  # TODO: in Julia 1.3 and later, we can just say I(n)
+    I3 = eye(3)
+    @test isapprox(tovoigt(II()), eye(6))
     @test isapprox(tovoigt(IT()), [I3  Z3;
                                    Z3  Z3])