Restrict valid schemes for LineLength/AllowURI

Per #1335, #1297, the existing way of matching URIs was too permissive
and allowed a lot of lines with URI-like content (e.g. Ruby namespaces)
to be ignored by the LineLength cop.

To work around this, a whitelist of schemes will stop the vast majority
of false positives.

URISchemes was added as a configuration option for flexibility.

Closes #1335

Fixed some missing test coverage that resulted from the above changes.
The configurable URISchemes caused an existing test to miss the line of
code it should have covered.
Merge pull request #1338 from smangelsdorf/restrict-uri-schemes-for-line-length-cop

Restrict valid schemes for LineLength/AllowURI
[Fix #1340] Handle call with no receiver correctly

For blocks with an unused argument which call a method with no explicit
receiver, the Style/SymbolProc would attempt to check 'receiver.type'
and crash because receiver is nil.