trappy/cache: Store the csv in utf-8 charset explicitly
Created by: credp
When we load the files from trace-cmd output, we explicitly choose to load them using utf-8. This means we may have characters in our trace which cannot be represented in ascii, which makes pandas upset when we try to store the parsed dataframes in the csv cache.
This is not an issue for Python3 versions, as the to_csv method defaults to utf-8, whilst the Python2 to_csv method defaults to ascii.
Signed-off-by: Chris Redpath email@example.com