Fix options Hash examples (:repository, :index)

Fix Readme.md to use correct symbol names for options Hash.

According to `base.rb` in v.1.2.6, the options Hash symbols are `:repository` instead of `:git_dir`, and `:index` instead of `:index_file`, as in:
https://github.com/schacon/ruby-git/blob/v1.2.6/lib/git/base.rb#L25
https://github.com/schacon/ruby-git/blob/v1.2.6/lib/git/base.rb#L72
Merge pull request #146 from guyhughes/patch-1

Fix symbols in options Hash examples (:repository, :index)
Moving version to 1.2.7
Updating History - v1.2.7
revparse: Supporting ref names containing SHA like substrings (40-hex strings) - closes #155
Fix warnings on Ruby 2.1.2.
Add 2.1 entry.
Merge pull request #157 from nishidayuya/fix_warnings_on_ruby_2.1.2

Fix File.exists? warnings on Ruby 2.1.2
Added a way to check if the repo is currently locked