Bump the version to 1.5.0

[kienstra]

Changes

• Bump the version to 1.5.0
• Add a CHANGELOG.md entry

Testing instructions

1. cd to the plugin root
2. ./bin/verify-versions.php
3. Expected: Success!

[dreamwhisper]

Looks good!

Version references:
{
    "README.md#stable-tag": "1.5.0",
    "genesis-custom-blocks.php#metadata": "1.5.0",
    "package.json": "1.5.0"
}
Success! All of the version numbers are the same, and there's a CHANGELOG.md entry. 

[kienstra]

@dreamwhisper, thanks a lot for testing and approving this!