Node 8 deprecation

Breaking changes

• Dropped node 8 compatibility

Fixes

• #90 Don't break when an unused variant points at a non-existent file (Andreas Lind)
• #87 Fix uniq-ification of strings that might contain non-UTF-16 chars (Andreas Lind)
• Silence pyftsubset output when checking for availability and it isn't installed
• Fix wrong size calculation when running subsetting using google fonts service