Specify JSONSchema spec

Since JSONSchema is still in a draft state as of this commit, we need to
specify a particular version of the JSONSchema spec

Signed-off-by: Ian Howell <ian.howell0@gmail.com>

cmd/helm/testdata/testcharts/chart-with-schema-negative/values.schema.yaml

@@ -1,3 +1,4 @@
+$schema: http://json-schema.org/draft-07/schema#
 title: Values
 type: object
 properties:

cmd/helm/testdata/testcharts/chart-with-schema/values.schema.yaml

@@ -1,3 +1,4 @@
+$schema: http://json-schema.org/draft-07/schema#
 title: Values
 type: object
 properties:

docs/charts.md

@@ -772,6 +772,7 @@ schema is the yaml representation of a [JSON Schema](https://json-schema.org/).
 It might look something like this:
 
 ```yaml
+$schema: http://json-schema.org/draft-07/schema#
 title: Values
 type: object
 properties:

pkg/chartutil/testdata/test-values.schema.yaml

@@ -1,3 +1,4 @@
+$schema: http://json-schema.org/draft-07/schema#
 title: Values
 type: object
 properties:

pkg/chartutil/values_test.go

@@ -492,6 +492,7 @@ chapter:
 
 func TestReadSchema(t *testing.T) {
 	schemaTest := `# Test YAML parse
+$schema: http://json-schema.org/draft-07/schema#
 title: Values
 type: object
 properties: