OSX build fixes + binaries from CI + Travis CI for OSX

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• OSX build fixes + use binaries from Gitlab CI
• Travis CI for OSX
• recompile libtezos so that it works with OSX 10.13 and newer
• Caching of OSX stuff for faster builds (half-done, needs more work)
• Fix post-accept issue with sockets in OSX (https://bugs.python.org/issue7995)
  • socket may be non-blocking after Accept on macOS dotnet/runtime#25069 and https://bugs.python.org/issue7995
• linker settings for OSX as part of the project (no need for the user to set it up)
• instructions for OSX in README (homebrew deps)

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SOLVED

There is still something weird going on with linking, because instead of using libtezos.dylib, the produced binaries try to use src/lib_ffi/ffi_main.so instead (I have to make a symlink or copy for it to work):

dyld: Library not loaded: src/lib_ffi/ffi_main.so

I have to figure that out and make a few more changes to build.rs so that it uses the .dylib extension when copying the lib file in OSX.

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Ok, OSX has this thing called install_name_tool, what I have to do is to update the dune file in tezos to correct libtezos-ffi.dylib and set the install_name to libtezos.dylib.