Fix 0.6 abstract type declaration depwarn (#102)

Author: yuyichao

REQUIRE

@@ -1,2 +1,2 @@
 julia 0.4
-Compat 0.9.1
+Compat 0.17.0

src/Calculus.jl

@@ -15,14 +15,14 @@ module Calculus
            second_derivative
 
     # TODO: Debate type system more carefully
-    # abstract BundledFunction
-    # abstract ScalarFunction
-    # abstract VectorFunction
-    # abstract ForwardDifference
-    # abstract CentralDifference
-    # abstract ComplexDifference
-    # abstract GradientEstimator
-    # abstract HessianEstimator
+    # abstract type BundledFunction end
+    # abstract type ScalarFunction end
+    # abstract type VectorFunction end
+    # abstract type ForwardDifference end
+    # abstract type CentralDifference end
+    # abstract type ComplexDifference end
+    # abstract type GradientEstimator end
+    # abstract type HessianEstimator end
 
     # typealias NonDifferentiableFunction Function
     # type DifferentiableFunction

src/symbolic.jl

@@ -12,8 +12,8 @@ import Base.show, Base.(==)
 #
 #################################################################
 
-abstract Symbolic
-abstract AbstractVariable <: Symbolic
+Compat.@compat abstract type Symbolic end
+Compat.@compat abstract type AbstractVariable <: Symbolic end
 Compat.@compat typealias SymbolicVariable Union{Symbol, AbstractVariable}