std::min<unsigned>, for VS2015

fixes #565
diff --git a/src/lib_json/json_value.cpp b/src/lib_json/json_value.cpp
index 9710f5b..694f32f 100644
--- a/src/lib_json/json_value.cpp
+++ b/src/lib_json/json_value.cpp
@@ -303,7 +303,7 @@
   // Assume both are strings.
   unsigned this_len = this->storage_.length_;
   unsigned other_len = other.storage_.length_;
-  unsigned min_len = std::min(this_len, other_len);
+  unsigned min_len = std::min<unsigned>(this_len, other_len);
   JSON_ASSERT(this->cstr_ && other.cstr_);
   int comp = memcmp(this->cstr_, other.cstr_, min_len);
   if (comp < 0) return true;
@@ -566,7 +566,7 @@
     char const* other_str;
     decodePrefixedString(this->allocated_, this->value_.string_, &this_len, &this_str);
     decodePrefixedString(other.allocated_, other.value_.string_, &other_len, &other_str);
-    unsigned min_len = std::min(this_len, other_len);
+    unsigned min_len = std::min<unsigned>(this_len, other_len);
     JSON_ASSERT(this_str && other_str);
     int comp = memcmp(this_str, other_str, min_len);
     if (comp < 0) return true;