remove $schema restrictions from JSON

[jkowalleck]

allowing $schema in JSON files was a good idea - see #106
but pinning it to a certain value - as if it was a namespace - was just wrong.

this pinning prevents reusing JSON documents that adhere to multiple schemas.

lets take the following JSON:

{
  "$schema": "http://cyclonedx.org/schema/bom-1.3.schema.json",
  "bomFormat": "CycloneDX",
  "specVersion": "1.3",
  "components": []
}

see also:

specification/schema/bom-1.5.schema.json

Lines 13 to 18 in 8af880d

	"$schema": {
	"type": "string",
	"enum": [
	"http://cyclonedx.org/schema/bom-1.5.schema.json"
	]
	},

This document IS valid to schema http://cyclonedx.org/schema/bom-1.3.schema.json.
This document WOULD also be valid to schema http://cyclonedx.org/schema/bom-1.5.schema.json, IF the $schema was removed from the document (since the $schema has an enum pining).

Let's make documents be able to be validated by upgraded schema-parsers.

The required change: remove the enum pinning from the $schema property.

[jkowalleck]

this change would also enable us to put $schema on our test data, without a need to change this with every bugfix/patch -- see #138