Fix Invalid examples

1. String#encode should not fail if given an invalid code like '\x81'
This is because this problem has been resolved by #41.

2. Net::LDAP#search should return a boolean if return_result is false.
Adopt Travis-CI
Add status image to README
Update README.rdoc

Fix typo
Add support version of ruby
Merge branch 'make_spec_green' of github.com:satoryu/ruby-net-ldap into make_spec_green
Change the value of source in Gemfile because deprecated
Merge pull request #59 from satoryu/make_spec_green

Make all specs green