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_test.go

@@ -0,0 +1,30 @@
+package mux
+
+import (
+	"context"
+	"net/http"
+	"testing"
+	"time"
+)
+
+func TestNativeContextMiddleware(t *testing.T) {
+	withTimeout := func(h http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			ctx, cancel := context.WithTimeout(r.Context(), time.Minute)
+			defer cancel()
+			h.ServeHTTP(w, r.WithContext(ctx))
+		})
+	}
+
+	r := NewRouter()
+	r.Handle("/path/{foo}", withTimeout(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		vars := Vars(r)
+		if vars["foo"] != "bar" {
+			t.Fatal("Expected foo var to be set")
+		}
+	})))
+
+	rec := NewRecorder()
+	req := newRequest("GET", "/path/bar")
+	r.ServeHTTP(rec, req)
+}