Travis improvements

[peternewman]

Add a spelling checker and fix the results it found. Also test on Mac (currently broken). Upstreaming some fixes originally in arduino-libraries/ArduinoModbus#12

[peternewman]

The Mac tests are failing due to this error in the user client:

[00][2E][00][00][00][06][FF][03][01][73][00][01]
Waiting for a confirmation...
<00><2E>Bytes flushed (9)
2/2 Adapted byte timeout (7ms > 5ms): 
Line 634: assertion error for 'rc == 1': 

[stephane]

Thank you for your script to add a spell check to Travis CI but I don't want to maintain a so complex script and Travis CI .org should be shut down in a few weeks (months I don't know).

I extracted the useful typo fixes from your PR.

[peternewman]

Thank you for your script to add a spell check to Travis CI but I don't want to maintain a so complex script

Fair enough. It did add lots of other things including a Mac build, but maybe that's more hassle than it's worth.

I've opened #586 which is a vastly simplified (but also more functional and improved) version of spell checking via GitHub actions instead (and it's found another new typo).

and Travis CI .org should be shut down in a few weeks (months I don't know).

They've been threatening that for at least five months now I think!

I extracted the useful typo fixes from your PR.

Thanks