Upgrade go-marathon to 15ea23e.

Our vendored copy contains a bug that causes unavailable Marathon nodes
to never be marked as available again due to a misconstruction in the
URL to the Marathon health check / ping endpoint used by go-marathon
internally.

A fix[1] has been published.

[1]https://github.com/gambol99/go-marathon/pull/283

vendor/github.com/gambol99/go-marathon/unreachable_strategy.go

@@ -0,0 +1,35 @@
+/*
+Copyright 2017 Rohith All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+	http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package marathon
+
+// UnreachableStrategy is the unreachable strategy applied to an application.
+type UnreachableStrategy struct {
+	InactiveAfterSeconds *float64 `json:"inactiveAfterSeconds,omitempty"`
+	ExpungeAfterSeconds  *float64 `json:"expungeAfterSeconds,omitempty"`
+}
+
+// SetInactiveAfterSeconds sets the period after which instance will be marked as inactive.
+func (us UnreachableStrategy) SetInactiveAfterSeconds(cap float64) UnreachableStrategy {
+	us.InactiveAfterSeconds = &cap
+	return us
+}
+
+// SetExpungeAfterSeconds sets the period after which instance will be expunged.
+func (us UnreachableStrategy) SetExpungeAfterSeconds(cap float64) UnreachableStrategy {
+	us.ExpungeAfterSeconds = &cap
+	return us
+}