Add Node 11 support

[xzyfer]

Fixes #2521

• wait for Node 11 on TravisCI
• wait for Node 11 on AppVeyor Node.js 11.1.0 appveyor/ci#2707
• make any necessary binding changes

[xzyfer]

Seeing a compilation failure on Linux builds

 g++-4.7 '-DNODE_GYP_MODULE_NAME=libsass' '-DUSING_UV_SHARED=1' '-DUSING_V8_SHARED=1' '-DV8_DEPRECATION_WARNINGS=1' '-D_LARGEFILE_SOURCE' '-D_FILE_OFFSET_BITS=64' '-DLIBSASS_VERSION="3.5.4"' -I/home/travis/.node-gyp/11.0.0/include/node -I/home/travis/.node-gyp/11.0.0/src -I/home/travis/.node-gyp/11.0.0/deps/openssl/config -I/home/travis/.node-gyp/11.0.0/deps/openssl/openssl/include -I/home/travis/.node-gyp/11.0.0/deps/uv/include -I/home/travis/.node-gyp/11.0.0/deps/zlib -I/home/travis/.node-gyp/11.0.0/deps/v8/include -I../src/libsass/include  -fPIC -pthread -Wall -Wextra -Wno-unused-parameter -m64 -O3 -fno-omit-frame-pointer -std=gnu++1y -std=c++0x -fexceptions -frtti -MMD -MF ./Release/.deps/Release/obj.target/libsass/src/libsass/src/ast.o.d.raw   -c -o Release/obj.target/libsass/src/libsass/src/ast.o ../src/libsass/src/ast.cpp
g++-4.7: error: unrecognized command line option ‘-std=gnu++1y’

[saper]

If gnu++1y is needed, then gcc 4.7 is too old.
https://github.com/nodejs/node/blob/v11.0.0/BUILDING.md#unixmacos says

gcc and g++ 4.9.4 or newer, or
clang and clang++ 3.4.2 or newer (macOS: latest Xcode Command Line Tools)

We need to keep the old compiler for the other systems. I wonder which systems we can no longer support on 11 due to this requirement.

[nschonni]

@saper I fixed that error by making sure it had GCC 4.9 for 11.
It does look like FreeBSD 10 was dropped for Node 11 https://nodejs.org/en/blog/release/v11.0.0/ so that might affect your binary builds

[saper]

Thanks, I noticed that - I am using 10 to build the modules but there is no problem to switch to 11 if needed. Fortunately a supported clang is available on both versions.

Yes, I have only seen the @xzyfer mails and after I checked the code I figured out it actually works. Thanks for fixing it.

[nschonni]

np, and it probalby makes sense to keep building <11 on FreeBSD 10 if you have the capacity to keep it around. I'm not really sure if that makes sense in terms of how people upgrade the distros in the FreeBSD world.

[xzyfer]

AppVeyor has added Node 11 support. We've begun the process of building the binaries.