Fix rtools_needed always returning custom

[burgerga]

Because the 'custom' entry in version_info has quite a large range and is checked first, rtools_needed always returned 'custom'. (as discussed in #97)

This fix:

• copies version_info to a local variable and removes custom
• sets r_version as function argument so the function can be unit tested
• adds unit tests for some frozen/bogus versions of R

All checks pass!

[jimhester]

Thanks! Can you please add a bullet to NEWS? It should briefly describe the change and end with (@yourname, #issuenumber).

[burgerga]

Done! (and rebased)

[jimhester]

Thanks!