Make PostAgent use WebRequestConcern, adding options for Basic Auth and User-Agent.
Check if every request has a correct Content-Type.
Add some specs to compensate for the removed specs for generate_uri.
Enable Graphviz on Heroku.

Graphviz is installed via heroku-buildpack-graphviz-src and the location
of dot(1) is automatically added to PATH.

There is also heroku-buildpack-graphviz-deb which should be easier to
install, but the Debian package installed has a font style problem.
Merge pull request #431 from knu/heroku-graphviz

Enable Graphviz on Heroku.
Merge pull request #429 from knu/post_agent_faraday

Make PostAgent use WebRequestConcern, adding options for Basic Auth and User-Agent.
Fix the link path of each badge.
upgrade RSpec
Merge pull request #435 from cantino/upgrade_rspec

upgrade RSpec