Fix github.com/sirupsen/logrus import path

Gopkg.lock

@@ -3,7 +3,7 @@
 
 [[projects]]
   digest = "1:69b1cc331fca23d702bd72f860c6a647afd0aa9fcbc1d0659b1365e26546dd70"
-  name = "github.com/Sirupsen/logrus"
+  name = "github.com/sirupsen/logrus"
   packages = ["."]
   pruneopts = "UT"
   revision = "bcd833dfe83d3cebad139e4a29ed79cb2318bf95"
@@ -276,7 +276,7 @@
   analyzer-name = "dep"
   analyzer-version = 1
   input-imports = [
-    "github.com/Sirupsen/logrus",
+    "github.com/sirupsen/logrus",
     "github.com/bwesterb/go-atum",
     "github.com/credentials/go-go-gadget-paillier",
     "github.com/getsentry/raven-go",

transport.go

@@ -13,9 +13,9 @@ import (
 	"strings"
 	"time"
 
-	"github.com/Sirupsen/logrus"
 	"github.com/go-errors/errors"
 	"github.com/hashicorp/go-retryablehttp"
+	"github.com/sirupsen/logrus"
 
 	"github.com/privacybydesign/irmago/internal/disable_sigpipe"
 	"github.com/privacybydesign/irmago/internal/fs"