requireNumericLiterals: Prevent erroring when first argument is an Identifier.

[rwjblue]

Prior to this fix, the following would trigger an error:

let foo = '123';
parseInt(foo, 16);

After this change, an error is only triggered when using an actual literal (as intended and indicated by the rule name).

Addresses https://github.com/DockYard/ember-suave/pull/80#issuecomment-183779369.

/cc @hzoo

[markelog]

Thanks

[hzoo]

Cool, thanks @rwjblue!