[install.sh] Correctly handle answers with multiple letters

[EtienneM]

emichon@biniou  ~/Documents/Scalingo  curl -O https://cli-dl.scalingo.com/install && bash install
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  4164    0  4164    0     0  41303      0 --:--:-- --:--:-- --:--:-- 41640
-----> Downloading Scalingo client...  DONE
-----> Extracting...   DONE
 /!\   Scalingo client is already installed (version 1.28.2)
       Do you want to replace it with version 1.28.2? [Y/n] q
       Invalid input, please enter 'y' or 'n': c
       Invalid input, please enter 'y' or 'n': x
       Invalid input, please enter 'y' or 'n': no
-----> Install Scalingo client to /usr/local/bin
       sudo required...