Changed HTTP URLs to HTTPS where possible (#270)

CMake/AbseilHelpers.cmake

CMake/README.md

CMakeLists.txt

LICENSE

README.md

absl/BUILD.bazel

absl/CMakeLists.txt

absl/algorithm/BUILD.bazel

absl/algorithm/CMakeLists.txt

absl/algorithm/algorithm.h

absl/algorithm/algorithm_test.cc

absl/algorithm/container.h

absl/algorithm/container_test.cc

absl/algorithm/equal_benchmark.cc

absl/base/BUILD.bazel

absl/base/CMakeLists.txt

absl/base/attributes.h

absl/base/bit_cast_test.cc

absl/base/call_once.h

absl/base/call_once_test.cc