travis: add go vet, addlicense checks; lower codecov threshold

- Temporarily disable caching of sources, it seems corrupted.
- Fix copyright on a few sources.
- Codecov is very aggressive about code coverage, reduce the
  expectations.

experimental/devices/mpu9250/accelerometer/accelerometer.go

@@ -1,3 +1,7 @@
+// Copyright 2020 The Periph Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
 package accelerometer
 
 // Valid accelerator values.

experimental/devices/mpu9250/reg/reg.go

@@ -1,3 +1,7 @@
+// Copyright 2020 The Periph Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
 package reg
 
 // Unsorted mix of register addresses, commands and constants.

experimental/devices/mpu9250/spi.go

@@ -1,3 +1,7 @@
+// Copyright 2020 The Periph Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
 package mpu9250
 
 import (