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
Fixed #70 - Support alias with bareword args
Don't run syntax check if run inside bundler

For some reason running `bundle exec rubocop`
slows down the start of the ruby interpreter (used with `-wc`)
tenfold. Until we figure what's causing this slowdown it's best
not to run syntax checks if we detect bundler's presence. See #64 for
more details.
Don't run example in the presence of bundler
Fix #64 - performance issue with Bundler
Clarify a bit encoding offence message
Add missing comma
Use the lexer instead of parser for alias
New cop EnsureReturn