add Ruby 2.1.0 for TravisCI
Fixed invalid byte sequence behavior
switch string-scrub gem to remove invalid bytes

Thank you for giving me nice information @repeatedly !!
ref. http://blog.livedoor.jp/sonots/archives/34702351.html
remove unnecessary output message on rake test

c.f. https://github.com/fluent/fluent-plugin-rewrite-tag-filter/pull/12#discussion_r10075183
release v1.4.1
add blog url for v1.4.1 release note
Update .travis.yml

c.f. http://blog.travis-ci.com/2014-04-28-upcoming-build-environment-updates/
Support log_level of Fluentd 0.10.43

ref. http://repeatedly.github.io/2014/02/new-log-level-parameter/
Load string-scrub gem under ruby < 2.1
Lock string-scrub version to v0.0.3
release v1.5.1