add a test for reported segfault on search_with_prefix
fix CI build

for head ruby removed test-unit
bump a new version for gem

fixed a segfault by updating hat-trie src
try to fix CI build on ruby-head
[still] trying to fix CI
CI: make sure ruby 2.1.0 works, ignore ruby-head
add correctness assertion to search_with_prefix
bump a new version for the fix