travis: fix grep of cppcheck.cppcheck log checking cppcheck code for errors

I'm sorry, I broke checking for errors in cppchecks own code.
I forgot to escape the quotation marks in the grep command which broke grepping for errors in the log.
Since I also negated the exit status to return false upon match, the error didn't cause the build to fail.

The build fails now because this
[test/testio.cpp:1138]: (style) Variable 'result_win64' is assigned a value that is never used.
is now successfully detected.