drop older Ruby versions for Travis (#310)
drop older Ruby versions for Travis (#310)

The older Ruby versions were dropped at a828a43

So, now it always fails at Ruby 2.0, 2.1 lanes on Travis CI, like

```
Bundler could not find compatible versions for gem "ruby":
  In Gemfile:
    ruby
    rmagick was resolved to 2.16.0, which depends on
      ruby (>= 2.2.0)
Could not find gem 'ruby (>= 2.2.0)', which is required by gem 'rmagick', in
any of the relevant sources:
  the local ruby installation
The command "bundle install --verbose" failed and exited with 6 during .
Your build has been stopped.
```

(See https://travis-ci.org/rmagick/rmagick/jobs/471690003#L2014)
rmagick/rmagick,rmagick/rmagick,rmagick/rmagick