Don't fail when unable to parse a file

When files don't end in .rb, we check if the
file has the ruby hashbang at the top

This commit rescues errors caused by empty or
unreadable files (EOFError) or unparseable
files (ArgumentError) such as invalid UTF-8
character string

Rubcop may now continue checking for ruby files
Update CHANGELOG
Refactor so rubocop finds no errors on itself
Merge pull request #67 from bf4/skip-invalid-files

Don't fail when unable to parse a file