CLI: Actual locally tested fix to race condition

Sorry for making new releases like this. It's that the live Browsh
services will only pull in officially released versions. But also that
this bug could affect a lot of people installing Browsh themselves.

Browsh CLI: fix for Marionette race condition

injectCustomCSS() fix, don't error of no body

Fix for Viper's lowercasing of config keys :/

See spf13/viper#635

Version 1.6.0

Update dependencies. Bump to v1.5.2

Merge 27826b3 into 883cd9c

Update JS and Go deps. Bump Browsh to v1.5.0

Update setup_linux_build_environment.md

Added webpack-cli installation.

Use thread-safe map for HTTP requests

This fixes a race condition when may parallel requests are made to the
HTTP service