Update macOS R-devel travis config for testing 4.0.0

[coatless]

This PR updates the toolchain to use:

• gfortran 8.2
  • https://github.com/fxcoudert/gfortran-for-macOS/releases/download/8.2/gfortran-8.2-Mojave.dmg
• New r-devel package endpoint for R 4.0.0
  • http://mac.r-project.org/high-sierra/R-4.0-branch/R-4.0-branch.pkg

As noted in:

https://stat.ethz.ch/pipermail/r-sig-mac/2020-April/013289.html

And addressed further in:

https://stat.ethz.ch/pipermail/r-sig-mac/2020-April/013311.html

---

What wasn't done is implement a pull of binaries available here:

http://mac.r-project.org/libs-4/

Note: There is a machine-readable list in http://mac.r-project.org/libs-4/INDEX

Would this be appreciated?

[coatless]

An error is being triggered due to this unit test:

travis-build/spec/build/script/r_spec.rb

Lines 78 to 79 in ab196ae

	should include_sexp [:cmd, %r{^curl.*mac\.r-project\.org/el-capitan/R-devel/R-devel-el-capitan\.pkg},
	assert: true, echo: true, retry: true, timing: true]

I'll change it to the appropriate URL that was updated above.

[jimhester]

LGTM

[jeroen]

@BanzaiMan lgtm 🚀

[jeroen]

Look good, thanks!

[BanzaiMan]

@jeroen Sorry, I overlooked this before merging #1901. Is it safe to merge this on top?

[jeroen]

@BanzaiMan yes that's safe, they only overlap a bit.