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.
[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.
Merge pull request #1338 from smangelsdorf/restrict-uri-schemes-for-line-length-cop

Restrict valid schemes for LineLength/AllowURI
Merge pull request #1341 from smangelsdorf/symbol-proc-crash

[Fix #1340] Handle call with no receiver correctly in Style/SymbolProc cop
Cut 0.26.1