Add of breaking is_executable check not documented in CHANGELOG

[GregOriol]

Hi,

In version 1.7.x, you added an is_executable($binary) check to the code, which may break (and does break) some setups, but you did not document it in the CHANGELOG. This has cause a serious outage to one of our services and it was difficult to find.

Please update the CHANGELOG to reflect important changes like this.

[SparkDragon]

I'm having the same issue because I'm using wkhtmltopdf as the binary path, as it is available globally in my PATH env var.

The recent update uses PHP's is_executable($binary), which returns false for global command names and only works with absolute or relative paths.

[Chris53897]

Related to #550

[clementvtrd]

I will update the CHANGELOG.md file right away, thank you to notify us. My apologies for the troubles

[KDederichs]

Thanks for adding it, I only also caught it cause it was complaining that it was restricted by the open_basedir. (Adding the binary here fixed it though)