Drop support for Go < 1.7: remove gorilla/context (#391)

* Drop support for Go < 1.7: remove gorilla/context
* Remove Go < 1.7 from Travis CI config
* Remove unneeded _native from context files

context.go

@@ -0,0 +1,22 @@
+package mux
+
+import (
+	"context"
+	"net/http"
+)
+
+func contextGet(r *http.Request, key interface{}) interface{} {
+	return r.Context().Value(key)
+}
+
+func contextSet(r *http.Request, key, val interface{}) *http.Request {
+	if val == nil {
+		return r
+	}
+
+	return r.WithContext(context.WithValue(r.Context(), key, val))
+}
+
+func contextClear(r *http.Request) {
+	return
+}