fix snowball stemmer bug in handling term `y's`

STEP 1c in line 828 checks `len(word) > 2` as last condition.
However, this will cause `string index out of range` Error
when stem the word `y's` since `word` will be `y` when we
come to line 828. Move the condition `len(word)>2` to beginning
will solve this issue.
Merge pull request #333 from cllu/patch-1

fix snowball stemmer bug in handling word `y's`