Updated minimum boost requirement on CMakeLists.txt.

Since Boost 1.66.0, Beast is a part of Boost.
So I removed Beast git submodule.

.gitmodules

@@ -1,3 +0,0 @@
-[submodule "external/beast"]
-	path = external/beast
-	url = https://github.com/boostorg/beast.git

CMakeLists.txt

@@ -96,22 +96,15 @@ IF ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC")
 ENDIF ()
 
 SET (Boost_USE_MULTITHREADED      ON)
-FIND_PACKAGE (Boost 1.59.0 REQUIRED COMPONENTS system)
+FIND_PACKAGE (Boost 1.66.0 REQUIRED COMPONENTS system)
 FIND_PACKAGE (Threads)
 
 IF(MQTT_BUILD_TESTS)
-    FIND_PACKAGE (Boost 1.59.0 REQUIRED COMPONENTS chrono timer unit_test_framework)
+    FIND_PACKAGE (Boost 1.66.0 REQUIRED COMPONENTS chrono timer unit_test_framework)
 ENDIF()
 
-IF (NOT MQTT_BEAST_INCLUDE_DIR)
-    IF (${Boost_VERSION} LESS 106600)
-        SET (MQTT_BEAST_INCLUDE_DIR "external/beast/include")
-    ENDIF ()
-ENDIF ()
-
 INCLUDE_DIRECTORIES (
     ${Boost_INCLUDE_DIR}
-    ${MQTT_BEAST_INCLUDE_DIR}
     include
 )