Remove '=delete' from template methods for Xcode 8

For Apple clang-800.0.42.1, which was released with Xcode 8 in
September 2016, the '=delete' on the 'is' and 'as' methods causes
the following errors for value.h:

  inline declaration of 'as<bool>' follows non-inline definition
  inline declaration of 'is<bool>' follows non-inline definition

etcetera for the other specializations of 'is' and 'as'.

Author: dgobbi

include/json/value.h

@@ -390,8 +390,8 @@ class JSON_API Value {
   bool isObject() const;
 
   /// The `as<T>` and `is<T>` member function templates and specializations.
-  template <typename T> T as() const = delete;
-  template <typename T> bool is() const = delete;
+  template <typename T> T as() const;
+  template <typename T> bool is() const;
 
   bool isConvertibleTo(ValueType other) const;