Fix FreeBSD requirements

Commit 72e53ae broke requirements check on FreeBSD systems that use
pkgng with the ports tree. This commit attempts to fix it by running
`pgk update` only on systems that actually use binary packages.
add extra case to find llvm instead of usual mri gcc checks, update #2418
Merge pull request #2420 from alexkay/master

Fix FreeBSD requirements
add extra check for target directory when migrating to gem-wrappers, update #2418
Merge branch 'master' of github.com:wayneeseguin/rvm
improve cleanup, add a message in installer it can be done, closes #2424, update #2418
Typo "rvm clean up all" should be "rvm cleanup all"
Merge pull request #2425 from elgalu/patch-1

Typo "rvm clean up all" should be "rvm cleanup all"
add a star for the new warning for cleanup