Fix bootstrap plugin

Actually it just makes simplecov think that all lines are tested.
I believe the best would be to write tests against paths that were added
instead of integrational for bootstrap so that we could check which paths
are added on different version (by stubbing gemspec.version message).

But, I must admit I'm mostly don't care about these out-of-the-box plugins.
And they are subjects of complete removeal in 1.0.0 release.
Fix RuboCop task
Allow pass URIs to liquid tag/filter

Related to #52
Ignore ruby-version
Allow pass custom attributes to liquid tag/filters

Closes #52
Move dev deps to Gemfile
Update HISTORY
Release v0.9.0