Fix Ruby warnings

[tute]

RUBYOPT=-w rake runs all tests with ruby warnings enabled, which are printed out in stderr. They are very noisy, as it shows warnings for every dependency in use alongside the tests output, so they are most useful when filtering only the warnings coming from this gem. We do that redirecting tests stdout to /dev/null (ignoring it), and redirecting stderr to stdout, with the following command:

RUBYOPT=-w rake 2>&1 1>/dev/null | grep merit | sort --unique

The output is:

/.../merit/lib/merit/controller_extensions.rb:50: warning: instance variable @user not initialized
/.../merit/lib/merit/controller_extensions.rb:50: warning: instance variable @user not initialized
/.../merit/test/dummy/app/models/merit/point_rules.rb:39: warning: ambiguous first argument; put parentheses or a space even after `-' operator

Merit should run without any warnings.

[yakschuss]

@tute - let me know if there's any further action I can take to help with this issue.

[tute]

Yes, thanks! Will close this issue in favor of your PR: #247.