Graphite module now takes $version as a parameter

Due to the way that the module was setup beforehand, we'd just install
_any_ version of graphite (carbon, graphite-web) from pip and not
check that we're installing compatible versions.

Due to the addition of the "package==version" string to the end of the
exec calls that we're using, the `$carbon_pip_args_str` and
`$graphite_pip_args_str` variables have been shortened so as not to
cause the PuppetLint tool to show warnings about lines being larger
than 80 lines.
Update README to explain that versions won't upgrade if incremented
Merge pull request #15 from gds-operations/use-fixed-package-versions

Graphite module now takes $version as a parameter