Upgrade server dependencies, manage them with govendor

vendor/github.com/xenolf/lego/acme/provider.go

@@ -0,0 +1,28 @@
+package acme
+
+import "time"
+
+// ChallengeProvider enables implementing a custom challenge
+// provider. Present presents the solution to a challenge available to
+// be solved. CleanUp will be called by the challenge if Present ends
+// in a non-error state.
+type ChallengeProvider interface {
+	Present(domain, token, keyAuth string) error
+	CleanUp(domain, token, keyAuth string) error
+}
+
+// ChallengeProviderTimeout allows for implementing a
+// ChallengeProvider where an unusually long timeout is required when
+// waiting for an ACME challenge to be satisfied, such as when
+// checking for DNS record progagation. If an implementor of a
+// ChallengeProvider provides a Timeout method, then the return values
+// of the Timeout method will be used when appropriate by the acme
+// package. The interval value is the time between checks.
+//
+// The default values used for timeout and interval are 60 seconds and
+// 2 seconds respectively. These are used when no Timeout method is
+// defined for the ChallengeProvider.
+type ChallengeProviderTimeout interface {
+	ChallengeProvider
+	Timeout() (timeout, interval time.Duration)
+}