Changing pagerduty_event_rule.catch_all field to Computed

[stmcallister]

PagerDuty API doesn't allow for catch_all to be set through the API. This PR makes that field Computed so that the provider doesn't try to manipulate its value. This is in response to issue 167

Test results

TF_ACC=1 go test -run "TestAccPagerDutyEventRule_Basic" ./pagerduty -v -timeout 120m

=== RUN   TestAccPagerDutyEventRule_Basic
--- PASS: TestAccPagerDutyEventRule_Basic (9.92s)
PASS
ok  	github.com/terraform-providers/terraform-provider-pagerduty/pagerduty	10.533s

TF_ACC=1 go test -run "TestAccPagerDutyEventRule_import" ./pagerduty -v -timeout 120m

=== RUN   TestAccPagerDutyEventRule_import
--- PASS: TestAccPagerDutyEventRule_import (5.21s)
PASS
ok  	github.com/terraform-providers/terraform-provider-pagerduty/pagerduty	5.787s

[heimweh]

Nice! This LGTM 👌