Update to 1.8.51

CHANGES:
1.8.51
    Add a test that unkwnon extensions are not touched
    Upgrade to haskell-src-exts-1.14
1.8.50
    Eliminate upper bounds on all dependencies
    #617, fix up notIn to take account of Template Haskell variables
    #573, suggest removing various deriving language extensions
1.8.49
    Remove ^^ ==> ** hint
    Remove a duplicate sqrt hint
    Ensure that --test failures throws an error
    Fix up the copyright year in --help
1.8.48
    Brackets at the root of annotations are fine
    Reduce a few more lambda expressions
1.8.47
    #613, compatibility with base-4.7