Handle case of multiple R_folders

In den original code, multiple folders resulted in error in turn.version.to.number 

error in strsplit(version_with_dots, "\\.") : non-character argument

because the length of R_folders was 1 (it was a list with 1 element, having  char[3]

This change was not back-tested against when there was only 1 folder.
Corrected wrong unlist position
Handled the case of using one directory for subsequent overwriting R installations by selecting the last one.
Merge pull request #10 from dmenne/master

Added workaround for multiple overwriting R installations in one directory