message: Update rhel-build.md docs about libsodium version
diff: ```
  cd /tmp
  - curl https://download.libsodium.org/libsodium/releases/libsodium-1.0.12.tar.gz | tar -xz
  - cd /tmp/libsodium-1.0.12
  + curl https://download.libsodium.org/libsodium/releases/libsodium-1.0.14.tar.gz | tar -xz
  + cd /tmp/libsodium-1.0.14
  ./configure
  make
  make install
  - rm -rf /tmp/libsodium-1.0.12
  + rm -rf /tmp/libsodium-1.0.14
  export PKG_CONFIG_PATH=$PKG_CONFIG_PATH:/usr/local/lib/pkgconfig
  export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/usr/local/lib
  ```
  + - versions of libsodium starting from 1.0.15 onwards miss the required `crypto_stream_aes128ctr_*` methods
  + - please note that in the future this release might be moved to subdirectory `old`
  1. Checkout and build the library:
  