Update client to v2.11.0 Release

[iancward]

Also switched from gopkg to retrieving package directly from github.com
Updating the client is pre-work prior to addressing #76

[iancward]

output from acceptance tests:

➜  terraform-provider-datadog git:(master) ✗ make testacc   
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v  -timeout 120m
?   	github.com/terraform-providers/terraform-provider-datadog	[no test files]

=== RUN   TestDatadogDowntime_import
Resource datadog_downtime.foo, type = datadog_downtime
--- PASS: TestDatadogDowntime_import (2.00s)
=== RUN   TestDatadogMonitor_import
--- PASS: TestDatadogMonitor_import (2.78s)
=== RUN   TestDatadogMonitor_import_no_recovery
--- PASS: TestDatadogMonitor_import_no_recovery (6.19s)
=== RUN   TestDatadogUser_import
--- PASS: TestDatadogUser_import (2.81s)
=== RUN   TestProvider
--- PASS: TestProvider (0.00s)
=== RUN   TestProvider_impl
--- PASS: TestProvider_impl (0.00s)
=== RUN   TestAccDatadogDowntime_Basic
Resource datadog_downtime.foo, type = datadog_downtime
--- PASS: TestAccDatadogDowntime_Basic (2.19s)
=== RUN   TestAccDatadogDowntime_BasicWithMonitor
Resource datadog_monitor.downtime_monitor, type = datadog_monitor
Resource datadog_downtime.foo, type = datadog_downtime
--- PASS: TestAccDatadogDowntime_BasicWithMonitor (2.62s)
=== RUN   TestAccDatadogDowntime_BasicMultiScope
Resource datadog_downtime.foo, type = datadog_downtime
--- PASS: TestAccDatadogDowntime_BasicMultiScope (1.88s)
=== RUN   TestAccDatadogDowntime_BasicNoRecurrence
Resource datadog_downtime.foo, type = datadog_downtime
--- PASS: TestAccDatadogDowntime_BasicNoRecurrence (1.50s)
=== RUN   TestAccDatadogDowntime_BasicUntilDateRecurrence
Resource datadog_downtime.foo, type = datadog_downtime
--- PASS: TestAccDatadogDowntime_BasicUntilDateRecurrence (1.44s)
=== RUN   TestAccDatadogDowntime_BasicUntilOccurrencesRecurrence
Resource datadog_downtime.foo, type = datadog_downtime
--- PASS: TestAccDatadogDowntime_BasicUntilOccurrencesRecurrence (2.68s)
=== RUN   TestAccDatadogDowntime_WeekDayRecurring
Resource datadog_downtime.foo, type = datadog_downtime
--- PASS: TestAccDatadogDowntime_WeekDayRecurring (1.94s)
=== RUN   TestAccDatadogDowntime_Updated
Resource datadog_downtime.foo, type = datadog_downtime
--- PASS: TestAccDatadogDowntime_Updated (3.39s)
=== RUN   TestAccDatadogDowntime_TrimWhitespace
Resource datadog_downtime.foo, type = datadog_downtime
--- PASS: TestAccDatadogDowntime_TrimWhitespace (1.41s)
=== RUN   TestResourceDatadogDowntimeRecurrenceTypeValidation
--- PASS: TestResourceDatadogDowntimeRecurrenceTypeValidation (0.00s)
=== RUN   TestResourceDatadogDowntimeRecurrenceWeekDaysValidation
--- PASS: TestResourceDatadogDowntimeRecurrenceWeekDaysValidation (0.00s)
=== RUN   TestAccDatadogMetricMetadata_Basic
--- PASS: TestAccDatadogMetricMetadata_Basic (2.30s)
=== RUN   TestAccDatadogMetricMetadata_Updated
--- PASS: TestAccDatadogMetricMetadata_Updated (3.26s)
=== RUN   TestAccDatadogMonitor_Basic
--- PASS: TestAccDatadogMonitor_Basic (2.03s)
=== RUN   TestAccDatadogMonitorServiceCheck_Basic
--- PASS: TestAccDatadogMonitorServiceCheck_Basic (1.39s)
=== RUN   TestAccDatadogMonitor_BasicNoTreshold
--- PASS: TestAccDatadogMonitor_BasicNoTreshold (2.02s)
=== RUN   TestAccDatadogMonitor_Updated
--- PASS: TestAccDatadogMonitor_Updated (5.96s)
=== RUN   TestAccDatadogMonitor_UpdatedToRemoveTags
--- PASS: TestAccDatadogMonitor_UpdatedToRemoveTags (5.19s)
=== RUN   TestAccDatadogMonitor_TrimWhitespace
--- PASS: TestAccDatadogMonitor_TrimWhitespace (1.46s)
=== RUN   TestAccDatadogMonitor_Basic_float_int
--- PASS: TestAccDatadogMonitor_Basic_float_int (5.10s)
=== RUN   TestAccDatadogTimeboard_update
--- PASS: TestAccDatadogTimeboard_update (7.80s)
=== RUN   TestValidateAggregatorMethod
--- PASS: TestValidateAggregatorMethod (0.00s)
=== RUN   TestAccDatadogUser_Updated
--- PASS: TestAccDatadogUser_Updated (3.78s)
PASS
ok  	github.com/terraform-providers/terraform-provider-datadog/datadog	73.170s

[masci]

LGTM!