Use https instead of http as far as possible in README

7 jobs for master in 19 minutes and 35 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #6490286
build

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docker windows 1809
build msvc amd64

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passed #6490289
docker windows 1809
build msvc x86

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passed #6490287
docker windows 1809
build msys2

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  Test
passed #6490292
doc-and-install

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passed #6490290
test

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passed #6490291
test valgrind

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