Remove the --auto option and update help text

The --auto option is run by default, there is no need to make it explicit.

• We remove the --auto option
• We also update the help text when running retest -h

# Old behaviour
    $ retest --auto
    $ retest --auto --all 

# New behaviour
    $ retest 
    $ retest --all

This release fixes:

• #116
• #117