Use environment variable to decide which rroonga to use

With this PR, when testing activegroonga, one can fetch and build rroonga from GitHub repo. With [local git repo](http://bundler.io/v1.3/git.html#local) feature of Bundler, she/he can choose local git repository.

I was surprised when activegroonga's test suddenly start failing with error ("make: *** No targets specified and no makefile found."). After looking at `test/run-test.rb`, I realized that the script tried to execute `make` to build rroonga from local repository which I recently cloned.

I thought that builing gem and resolving load path are not test's job. They should be handled with Bundler/RubyGem and by developer. So, I propose this PR.