gem-config: add semian

[ento]

Motivation for this change

Building the semian gem with bundlerEnv results in this error on Ubuntu/Debian because it needs openssl

error output

building '/nix/store/m8k2p5dczdrnhsar4hd0v7m4abyz27h0-ruby2.3.8-semian-0.8.6.drv'...
unpacking sources
patching sources
configuring
no configure script, doing nothing
installing
buildFlags: 
WARNING:  You build with buildroot.
  Build root: /
  Bin dir: /nix/store/g0jqf0h066an4i9zmq0jq7rd9sxgsygf-ruby2.3.8-semian-0.8.6/lib/ruby/gems/2.3.0/bin
  Gem home: /nix/store/g0jqf0h066an4i9zmq0jq7rd9sxgsygf-ruby2.3.8-semian-0.8.6/lib/ruby/gems/2.3.0
Building native extensions. This could take a while...
ERROR:  Error installing /nix/store/gj5is0zci89d3ldlqpjw5li9kbrvl0k5-semian-0.8.6.gem:
        ERROR: Failed to build gem native extension.

    current directory: /nix/store/g0jqf0h066an4i9zmq0jq7rd9sxgsygf-ruby2.3.8-semian-0.8.6/lib/ruby/gems/2.3.0/gems/semian-0.8.6/ext/semian
/nix/store/nw0k7lw6i79bij8rp0l6v9rqjm4j7386-ruby-2.3.8/bin/ruby -r ./siteconf20190125-12320-o96lio.rb extconf.rb
checking for openssl/sha.h... no
openssl is missing. please install openssl.
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
        --with-opt-dir
        --without-opt-dir
        --with-opt-include
        --without-opt-include=${opt-dir}/include
        --with-opt-lib
        --without-opt-lib=${opt-dir}/lib
        --with-make-prog
        --without-make-prog
        --srcdir=.
        --curdir
        --ruby=/nix/store/nw0k7lw6i79bij8rp0l6v9rqjm4j7386-ruby-2.3.8/bin/$(RUBY_BASE_NAME)

To see why this extension failed to compile, please check the mkmf.log which can be found here:

  /nix/store/g0jqf0h066an4i9zmq0jq7rd9sxgsygf-ruby2.3.8-semian-0.8.6/lib/ruby/gems/2.3.0/extensions/x86_64-linux/2.3.0/semian-0.8.6/mkmf.log

extconf failed, exit code 1

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions: Debian 9.5
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

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