add support for a specific version

By default, the module will continue to ensure the package is installed, but
a user may configure a specific version or specify 'latest' to ensure
instead.
Move .gemfile to Gemfile, update Gemfile contents

Having a hidden gemfile is very unintuitive, and isn't commonly done.
This commit moves the Gemfile so that `bundle install` will work by
default.
Update README with testing information
Add the travis CI build status link to the README
Merge pull request #46 from adrienthebo/gemfile-testing

Gemfile testing
Merge pull request #47 from adrienthebo/readme-travis-status

Add the travis CI build status link to the README
Run rspec tests on ruby 1.9.3 on travis-ci
Merge pull request #48 from adrienthebo/test-ruby-193

Run rspec tests on ruby 1.9.3 on travis-ci
Merge pull request #45 from maestrodev/select_package_version

add support for a specific version
Fixed local variable deprecation warnings

Before this patch local variables that appeared in the
mongod.conf.erb template were not prefixed with an '@'. This
style is deprecated and produces warnings. This commit fixes
those warnings.
Merge pull request #50 from hogepodge/warnings

Fixed local variable deprecation warnings