Validate that the sudo file is valid

This is pretty important, as you can easily hose your system by messing up your sudoers file, and have to go through hoops to fix it (as I learnt the hard way...)
Spec for new `validate_cmd` value
Lint Fixes:

manifests/sudoers.pp - WARNING: variable not enclosed in {} on line 68
manifests/sudoers.pp - WARNING: variable not enclosed in {} on line 77
manifests/sudoers.pp - WARNING: indentation of => is not properly aligned on line 73
Add stdlib to fixtures
Change Travis to standard matrix
Change Gemfile to standard
Add stdlib to Modulefile
Use template for Rakefile
Moving validate_cmd into new file
Moving validation back into sudo::sudoers class

Also adding basic validate_cmd from stdlib
Add logic around checking Puppet version

Without this, the spec will fail with `Invalid parameter validate_cmd`
Adds ~> 3.6.0 to Travis matrix

As we want to explicitly test the validate_cmd stuff
Lint indentation fixes
Merge pull request #18 from petems/add_sudo_validate

Add sudo validate