Replaced Travis CI with Github Actions

[guruofquality]

/usr/bin/ld: CMakeFiles/SoapySDRServer.dir/ServerListener.cpp.o: undefined reference to symbol 'pthread_create@@GLIBC_2.2.5'

Could have sworn I was linking to pthread.

Should I merge this an expect another pr?

[ncorgan]

I don't have another branch at this point yet, but that's the intent.