Don't require compiler field in fpm.toml

[milancurcic]

Following up on the discussion in #60, let's remove the requirement for the compiler setting in the package-specific fpm.toml.

For now, FPM should default to gfortran, and we can put a note in the README or the packaging guide that gfortran is currently the only compiler supported. We can relax this at a later time. The compiler setting can still remain in the toml parser as optional--perhaps we will want to allow package-specific choice of compiler.

[everythingfunctional]

That's reasonable for now. I'll make that change on my PR