Use "builtin cd" instead of just "cd"

[andsild]

Similar to #68

In case someone has overloaded the "cd" command (with alias), the
plugin's behaviour can be odd. E.g. if a cd alias also types in "ls"
after "cd", the autocompletion will spew out directory contents each
time user hits tab.

This MR amends said problem by ensuring cd is default version.

[mosabua]

Does this work for the supported shells?

[glhez]

What are the supported shells?

Given the name of the project, file and so, I would assume it is "bash".
If not, then you perhaps you should update the README.md to indicate what is supported.

builtin and cd are indeed bash builtin (sorry, there are no direct link the exact location)

1. cd
2. builtin

By the way my PR#68 should have used the command builtin which is clearly more obvious than \.