Update module for Terraform 2.6 compatibility

[ghost]

The main sticking points for our upgrade were resolving the (apparently) now conflicting role_arn and destination_arn parameters for the aws_cloudwatch_log_subscription_filter (hence the aws_lambda_permission solution), and de-conflicting the IAM role name when creating multiple copies of the module.

Obviously we're still tied to the us-east-1 region due to the s3_bucket and s3_key arguments to the aws_lambda_function, but this is a bit closer to a multi-region solution.

[krisread]

Hey @mattzimmerman @binarylogic

Any chance this is ready?

Am blocked right now on using the lambda.

Instructions on the timber.io website do not work.

Defining the module multiple times causes errors... I presume this is the fix.

[krisread]

FYI I tried this patch by checking out this commit and telling terraform to use my fork

Doesn't seem to fix the problem with "multiple copies of the module" not being allowed.

An module call named "timber_log_forwarding" was already defined at
timber-io.tf:34,1-31. Module calls must have unique names within a module.

Even giving each module a name= parameter, the latest terraform doesn't seem to ever allow you to define the same module twice.