Set default versions for to build Nodejs 8.11.1 and Yarnpkg 1.6.0

[nsaechao]

Summary:

• Remove building for Mac versions < 10.8.
• Fix a bug in the build-node.sh script where the default version was incorrectly set with a '-' character.

Tested:

./build-node.sh & ./build-yarnpkg.sh

[nsaechao]

Based on the new format for merging and then uploading binaries, what is the process if there are no script changes required to add a new binary?

[stuhood]

Based on the new format for merging and then uploading binaries, what is the process if there are no script changes required to add a new binary?

The new process would be to copy the edited script into the locations where the binaries should be fetched, and then create symlinks to the other locations where the binary should be used.

See #66 for an example.