add Bundler.with_clean_env

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.
remove 'RUBYLIB' and 'GEM_HOME' environments

- remove these variables when executing analyzers
- these variables may cause an error when using a different version of
  ruby from the one used by OACIS
refctoring using Bundler.clean_system
Merge pull request #272 from crest-cassia/analyzer_with_clean_env

add Bundler.with_clean_env