Fix Encoding::UndefinedConversionError exceptions

Use Rugged::Blob#text to get the content of a file,
and assume we only have text dependency files (ie. non-binary files).

Rugged::Blob#content returns ASCII-8BIT encoded strings but this may be an issue
when JSON.generate tries to convert these strings to UTF-8 encoding;
some caracters can't be converted to UTF-8, like \xC3.
Bump version 0.2.2