Make .clang-format consistent with cpplint (#435)

## Changes
* Apparently, there were some changes to `cpplint`, and it now expects gtest headers to go above C++ std headers.
* This PR reorders the headers and removes custom ordering from .clang-format.

## Examples
* `./lint.sh` no longer produces any error messages.

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.clang-format

@@ -6,18 +6,5 @@ BinPackArguments: false
 BinPackParameters: false
 ColumnLimit: 120
 DerivePointerAlignment: false
-IncludeCategories:
-  - Regex:           '^<ext/.*\.h>'
-    Priority:        2
-    SortPriority:    0
-  - Regex:           '^<.*\.h>'
-    Priority:        2
-    SortPriority:    0
-  - Regex:           '^<.*'
-    Priority:        1
-    SortPriority:    0
-  - Regex:           '.*'
-    Priority:        3
-    SortPriority:    0
 Standard: c++17
 ...

libSetReplace/test/Set_test.cpp

@@ -1,9 +1,9 @@
 #include "Set.hpp"
 
-#include <vector>
-
 #include <gtest/gtest.h>
 
+#include <vector>
+
 #include "Match.hpp"
 #include "Rule.hpp"