Fix nodejs and yarn checks

[aganov]

webpacker:check_node task is calculating nodejs version wrong

-> % bundle exec rake webpacker:check_node
Webpacker requires Node.js >= 6.4 and you are using v6.10.2
Please upgrade Node.js https://nodejs.org/en/download/

This PR replaces calculation to use Gem::Version instead of version.to_f. Also rails is using patched version of backticks which is not raising exception if the binary is not exists. More info can be found here: Object#` I've added a fix to webpacker:check_node and webpacker:check_yarn to raise Errno::ENOENT if the version check failed with blank value. I'm not sure if we have to raise and rescue in case of patched backticks in active_support, but I'm leaving it like this so it will work if the patch is removed.