Open source files in binary mode

Opening non-UTF-8 files as UTF-8 succeeds in some cases and will throw
<ArgumentError: invalid byte sequence in UTF-8> when comparing against a
regexp. Forcing Ruby to use binary mode will keep strings as ASCII-8BIT
and seems to work on all current versions of Ruby.

Moreover, there is no way to know for sure which encoding is used
without heuristics or parsing the potential encoding shebang thus binary
mode is probably safer.