Add rbenv versions --skip-aliases option

Useful in combination with `--bare` to list just the unique version
numbers without the extra directory entries that are symlinks to other
version numbers in the same directory.

test/versions.bats

@@ -139,3 +139,18 @@ OUT
 1.8.7
 OUT
 }
+
+@test "doesn't list symlink aliases when --skip-aliases" {
+  create_version "1.8.7"
+  ln -s "1.8.7" "${RBENV_ROOT}/versions/1.8"
+  mkdir moo
+  ln -s "${PWD}/moo" "${RBENV_ROOT}/versions/1.9"
+
+  run rbenv-versions --bare --skip-aliases
+  assert_success
+
+  assert_output <<OUT
+1.8.7
+1.9
+OUT
+}