Fix specified version of Plek

The version of Plek specified as the minimum requires an argument to be passed
to Plek.new, but we call it in component_resolver.rb without an argument.

Passing an argument was made optional in Plek 1.0.0, but 1.0.0 requires an
environment variable to be set. 1.1.0 and above don't, so lets specify that.
Merge pull request #95 from alphagov/fix_plek_specification

Fix specified version of Plek