Remove gorilla/context as part of pre-1.7 support (#114)

Author: elithrar

.travis.yml

@@ -1,7 +1,6 @@
 module github.com/gorilla/csrf
 
 require (
-	github.com/gorilla/context v1.1.1
 	github.com/gorilla/securecookie v1.1.1
 	github.com/pkg/errors v0.8.0
 )

context_legacy.go

@@ -0,0 +1,4 @@
+github.com/gorilla/securecookie v1.1.1 h1:miw7JPhV+b/lAHSXz4qd/nN9jRiAFV5FwjeKyCS8BvQ=
+github.com/gorilla/securecookie v1.1.1/go.mod h1:ra0sb63/xPlUeL+yeDciTfxMRAA+MP+HVt/4epWDjd4=
+github.com/pkg/errors v0.8.0 h1:WdK/asTD0HN+q6hsWO3/vpuAkAr+tw6aNJNDFFf0+qw=
+github.com/pkg/errors v0.8.0/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=