add Gemfile to project since jekyll gem is invoked by default grunt task

since we’re using jekyll to compile the github pages, let’s add
it as a tool in the listed dependencies so contributors don’t
have to guess.

the following is my terminal output before adding the Gemfile

$ grunt

Running "recess:bootstrap" (recess) task
File "dist/css/bootstrap.css" created.

Running "recess:min" (recess) task
File "dist/css/bootstrap.min.css" created.
Original: 125095 bytes.
Minified: 102472 bytes.

Running "recess:theme" (recess) task
File "dist/css/bootstrap-theme.css" created.

Running "recess:theme_min" (recess) task
File "dist/css/bootstrap-theme.min.css" created.
Original: 19464 bytes.
Minified: 17493 bytes.

Running "jshint:gruntfile" (jshint) task
>> 1 file lint free.

Running "jshint:src" (jshint) task
>> 12 files lint free.

Running "jshint:test" (jshint) task
>> 13 files lint free.

Running "qunit:files" (qunit) task
Testing js/tests/index.html Starting test suite
================================================

✔ All tests passed in 'transition' module
✔ All tests passed in 'alert' module
✔ All tests passed in 'button' module
✔ All tests passed in 'carousel' module
✔ All tests passed in 'collapse' module
✔ All tests passed in 'dropdowns' module
✔ All tests passed in 'modal' module
✔ All tests passed in 'scrollspy' module
✔ All tests passed in 'tabs' module
✔ All tests passed in 'tooltip' module
✔ All tests passed in 'popover' module
✔ All tests passed in 'affix' module

================================================
Tests completed in 2238 milliseconds
216 tests of 216 passed, 0 failed.
OK
>> 216 assertions passed (2238ms)

Running "jekyll:docs" (jekyll) task
`jekyll build` was initiated.

Jekyll output:
Warning: Command failed: rbenv: jekyll: command not found

The `jekyll' command exists in these Ruby versions:
  2.0.0-p247

 Use --force to continue.

Aborted due to warnings.