Update Gemfile to reflect changes in Bundler

Use explicit `source`
Fix incorrect names for packages in recipes/ebs

Use `value_for_platform` to with platform-specific package names for libxslt and libxml

Closes #77
[#77] Simplify recipes/ebs code to use `value_for_platform_family`
[#77] Added `build-essential` into Vagrantfile and recipes/ebs

NOTE: Even though the "build-essential" cookbook is included in the node `run_list`,
`build-essential` is not installed and `make` is not available before the Chef run,
making the installation of the Fog gem fail.

That's why we need to include the build-essential/make/gcc/etc packages also in the
`value_for_platform_family` in recipes/ebs.
Release 0.2.7