Problem: travis saucelabs is complaining about Safari timeouts

Problem - Safari timed out when running tests via SauceLabs/Travis

For reasons not well understood, Selenium fails to attach to Safari 11 when running MacOS Sierra. The fix is simple, update the SauceLabs configuration to use High Sierra instead. This is achieved by editing wct.conf.js

See this commit for an example: https://github.com/predixdesignsystem/px-map/commit/5b75c0eb0481cee24a88564fa80cde619a7c2509#diff-17eabf568535c21ce18e431892b19678

Also note that we only support the last two major releases of Safari. If Safari 10 is in wct.conf.js, change it to 11 (and add 12 if it's missing)