When b:sy is present, only disable Sy if the file disappears
When b:sy is present, only disable Sy if the file disappears

In #344, we disable Sy if any condition in s:skip() is true.  This is
too aggressive.  If b:sy was already set then we should prefer to leave
it set, except when we can't access the file anymore.

Therefore, stop using s:skip() to handle this condition and simply check
filereadable().

Closes #347

mhinz/vim-signify