Updates for compatibility with recent Arduino IDE versions

[per1234]

arduino-mavlink no longer compiles with recent versions of the Arduino IDE for multiple reasons. This pull request resolves these issues.

• Fix iincorrect FastSerial library include order.
• Use correct progmem types. The types previously used are no longer supported by recent versions of avr-gcc.
• Fix invalid library warning for mavlink library.

I don't own the hardware to be able to test these changes beyond verifying that they do now compile. I started the work with the understanding that someone who owns the hardware would do the testing but this ended up being beyond their capabilities. However, these are pretty basic changes unlikely to cause any problems.