compute_firewall: set source_ranges to Computed to avoid perpetual diff

[pmcatominey]

If this is not set in Terraform, then the API defaults to 0.0.0.0/0,
Terraform would then attempt to remove that value and so on.

Fixes #108

TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run TestAccComputeFirewall_ -timeout 120m
?   	github.com/terraform-providers/terraform-provider-google	[no test files]
=== RUN   TestAccComputeFirewall_importBasic
--- PASS: TestAccComputeFirewall_importBasic (72.87s)
=== RUN   TestAccComputeFirewall_basic
--- PASS: TestAccComputeFirewall_basic (65.87s)
=== RUN   TestAccComputeFirewall_update
--- PASS: TestAccComputeFirewall_update (88.22s)
=== RUN   TestAccComputeFirewall_noSource
--- PASS: TestAccComputeFirewall_noSource (79.78s)
PASS
ok  	github.com/terraform-providers/terraform-provider-google/google	306.762s

[paddycarver]

Code makes sense to me, tests pass. 👍 Thanks @pmcatominey!

[ghost]

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