resource/aws_lambda_layer_version: Use Terraform 0.11.12 and later co…

…mpatible file hashing function in documentation and testing

The updated file hashing function is forwards compatible with Terraform 0.12, which does not allow the use of the `file()` function with binary file content.

Previous output from Terraform 0.12 acceptance testing:

```
--- FAIL: TestAccAWSLambdaLayerVersion_update (1.73s)
    testing.go:568: Step 0 error: config is invalid: Error in function call: Call to function "file" failed: contents of test-fixtures/lambdatest.zip are not valid UTF-8; to read arbitrary bytes, use the filebase64 function instead.
```

Output from Terraform 0.11.12 acceptance testing:

```
--- PASS: TestAccAWSLambdaLayerVersion_update (26.10s)
```

Output from Terraform 0.12 acceptance testing:

```
--- PASS: TestAccAWSLambdaLayerVersion_update (25.75s)
```

aws/resource_aws_lambda_layer_version_test.go

@@ -330,7 +330,7 @@ func testAccAWSLambdaLayerVersionCreateBeforeDestroy(layerName string, filename
 resource "aws_lambda_layer_version" "lambda_layer_test" {
   filename         = "%s"
   layer_name       = "%s"
-  source_code_hash = "${base64sha256(file("%s"))}"
+  source_code_hash = "${filebase64sha256("%s")}"
 
   lifecycle {
     create_before_destroy = true

website/docs/r/lambda_layer_version.html.markdown

@@ -45,7 +45,7 @@ large files efficiently.
 * `compatible_runtimes` - (Optional) A list of [Runtimes][2] this layer is compatible with. Up to 5 runtimes can be specified.
 * `description` - (Optional) Description of what your Lambda Layer does.
 * `license_info` - (Optional) License info for your Lambda Layer. See [License Info][3].
-* `source_code_hash` - (Optional) Used to trigger updates. Must be set to a base64-encoded SHA256 hash of the package file specified with either `filename` or `s3_key`. The usual way to set this is `${base64sha256(file("file.zip"))}`, where "file.zip" is the local filename of the lambda layer source archive.
+* `source_code_hash` - (Optional) Used to trigger updates. Must be set to a base64-encoded SHA256 hash of the package file specified with either `filename` or `s3_key`. The usual way to set this is `${filebase64sha256("file.zip")}` (Terraform 0.11.12 or later) or `${base64sha256(file("file.zip"))}` (Terraform 0.11.11 and earlier), where "file.zip" is the local filename of the lambda layer source archive.
 
 ## Attributes Reference