PARQUET-1138: Fix Arrow 0.7.1 build

This is a very minor issue with the 1.3.1 RC0. If this build passes cleanly I will vote to approve the release as this only affects this unit test

Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #410 from wesm/arrow-0.7.1-fix-build and squashes the following commits:

fd6a527 [Wes McKinney] Add comment
f95ff0b [Wes McKinney] Fix compilation with Arrow 0.7.1, set 0.7.1 in ThirdpartyToolchain.cmake
diff --git a/cmake_modules/ThirdpartyToolchain.cmake b/cmake_modules/ThirdpartyToolchain.cmake
index 3961abd..a470fc1 100644
--- a/cmake_modules/ThirdpartyToolchain.cmake
+++ b/cmake_modules/ThirdpartyToolchain.cmake
@@ -366,7 +366,7 @@
     -DARROW_BUILD_TESTS=OFF)
 
   if ("$ENV{PARQUET_ARROW_VERSION}" STREQUAL "")
-    set(ARROW_VERSION "8309556c7d2b0e14df1422baa574cf2de8c1bd3b")
+    set(ARROW_VERSION "0e21f84c2fc26dba949a03ee7d7ebfade0a65b81")  # Arrow 0.7.1
   else()
     set(ARROW_VERSION "$ENV{PARQUET_ARROW_VERSION}")
   endif()
diff --git a/src/parquet/arrow/arrow-reader-writer-test.cc b/src/parquet/arrow/arrow-reader-writer-test.cc
index fc6410d..a18c565 100644
--- a/src/parquet/arrow/arrow-reader-writer-test.cc
+++ b/src/parquet/arrow/arrow-reader-writer-test.cc
@@ -951,7 +951,7 @@
 
   std::shared_ptr<Array> expected_values =
       std::make_shared<::arrow::NullArray>(SMALL_SIZE);
-  AssertArraysEqual(*expected_values, *chunked_array->chunk(0));
+  internal::AssertArraysEqual(*expected_values, *chunked_array->chunk(0));
 }
 
 template <typename T>