Allow gem executable to contain spaces

rbenv has a special exec function which loads the correct ruby version before executing a command.
The syntax for this is `/usr/local/rbenv/bin/rbenv exec gem ...` for example.
But previously when specifying executable='/usr/local/rbenv/bin/rbenv exec gem' this would not work because the string was treated as one executable. This PR fixes that by splitting the executable on spaces.
Merge pull request #7801 from koenpunt/npm-rbenv-exec

Allow gem executable to contain spaces