Use `Coco::ROOT` constant instead of $COCO_PATH global

The use of a global variable hinders JRuby ability to create optimized
code. And globals are just just plain bad, aren't they? :)
Fix whitespace
Remove Gemfile.lock, this is a library

`Gemfile.lock` should should not be checked in for gems.

See the article "Clarifying the Roles of the .gemspec and Gemfile" [1] from
one of the authors of Bundler.

[1]: http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/
Use Bundler to build and install the gem

Bundler is already used by coco (see the `Gemfile`) and is the standard
tool for all gem-related things. Let's use the provided Bundler
Rake tasks instead of rolling our own.
Integrate default Bundler .gitignore
Remove custom bundler command fro Travis

Bundler is already declared as a development dependency
Merge branch 'gioele-gemspec-cleanup' into development
Remove duplicated files
Merge branch 'fix-gemspec' into development
Merge branch 'no-globals' of github.com:gioele/coco into gioele-no-globals
Merge branch 'gioele-no-globals' into development
Delete the right file!
Merge branch 'fix-random-failing-test' into development
Remove the CLI garbage

At the end of the test suite, coco is running, checking the coverage of
coco itself. To avoid garbage on the command line, I create a .coco.yml
which excludes the lib/ folder at the end of the test suite.
Merge branch 'avoid-garbage' into development