Correct pkg-config to pkgconfig for EL 7

Prior to this commit, `rvm requirements` attempted to bring in
`pkg-config` for EL 7. However, this package doesn't exist; the
correct package name is `pkgconfig`.

This commit renames the package for Red Hat 7, and allows Rubies to be
compiled successfully.
Merge pull request #3101 from rji/el7-pkgconfig-fixup

Correct pkg-config to pkgconfig for EL 7