handlebars-lang / handlebars.js

fixes #1673

- we no longer need to pin to yargs 14 for node 6 & 8 support

- demonstrates the regression

- make sure we're aware of all compatibility issues

- handlebars is changing to a policy of only supporting current and active versions of Node.js

This reverts commit c2f1e62.

- indicate changes have been reverted

This reverts commit 309d2b4.

- allows us to support Node v6 and v8 for now

- drop support for versions less than Node v6 via engines
- update integration tests to also test newer versions

This reverts commit 2fd4a3d.

- Add spec/tmp directory with .gitkeep file to indicate the folder is intentional
- Add the folder contents to .gitignore
- Use this folder to output the sourcemap file during bin tests. This file is a sideeffect of the main test

This reverts commit 738391a.
Think this needs to be left to the release tool :|

fixes #1548

- add a guard to show readable syntax error for if / unless helper
- prevents against 3 different errors that can be generated by different systax errors