add Bundler.with_clean_env

fixed #271 

Analyzer process is contaminated by BUNDLER_... environment variables, which may cause a problem when executing an analyzer implemented in Ruby.
To avoid this problem, analyzer is called in Bundler.with_clean_env block.
Bundler.clean_systemというコマンド実行までしてくれるメソッドがあるようです．

``` diff
        cmd = "(#{arn.analyzer.command}) 1> _stdout.txt 2> _stderr.txt"
+        envs_to_remove = {'RUBYLIB' => nil, 'GEM_HOME' => nil} # These variables are not cleaned by with_clean_env
+        Bundler.clean_system(envs_to_remove, cmd)
        unless $?.to_i == 0
          raise "Rc of the analyzer is not 0, but #{$?.to_i}"
        end
```
ではどうでしょうか？