Replacing --rules-dir option with --rule-files and --interfaces-files

This is done to get rid of dependency on the directory tree structure.
It assumes the you would have rules_dir/env/component.rb
rules_dir/env/interfaces.yaml and rules_dir/common_component.rb files.
This forces end user to adhere to the tree structure, which wasn't a
good practice.

Now you would be able to specify --rules-files file1.rb,xyz/file2.rb,..
and --interfaces-files ifile1.yaml,abc/ifile2.yaml
Removed option environment

Now as we have removed the dependency of tree structure, we can remove
environment option from command line interface as it doesn't add value
anymore
Updated README as per the new options
Updated options --rule-files --rules-files
Fixing integration tests

Forgot about the integration test with removal of -e option, fixing them
in this commit
Merge pull request #7 from alphagov/removing_tree_structure_dependency

Removing tree structure dependency