[#51001091] Remove rbenv shims when a .ruby-version file isn't present

Due to the way that cucumber works[1], having rbenv's shims on the
PATH causes a `ruby: no Ruby script found in input (LoadError)` error
to occur.

This fix removes rbenv's shims from the PATH so that the provided (our
packaged) version of Ruby (1.9.2) gets used.

[1] https://github.com/cucumber/cucumber/blob/281b271f8e1957b36eee357a4c90727b91ced65e/lib/cucumber/rake/task.rb#L89
Merge pull request #81 from alphagov/remove-rbenv-shims

[#51001091] Remove rbenv shims when a .ruby-version file isn't present