GemRequirement.py: cmd needs a space after -i

https://gitlab.com/coala/package_manager/blob/master/dependency_management/requirements/GemRequirement.py#L58

cmd = 'gem list -i' + self.package

this causes GemRequirement("img_checker").is_installed() return False when it is installed.

I have tried this in python shell and running that gives me false, where if I run GemRequirment(" img_checker") it returns True

This will also cause error for other gem files i think

>>> GemRequirement('rubocop').is_installed()
False
>>> GemRequirement('reek').is_installed()
False