Workaround for HList compilation problems in Scala 2.10.3, 2.11.0-M7.

Related to issue #577.

Changing hconsShape as per Paul's suggestion in
https://issues.scala-lang.org/browse/SI-8146 avoids the spurious type
errors and non-termination issues in the minimized test case.
Merge pull request #609 from slick/tmp/hcons-shape-workaround

Workaround for HList compilation problems in Scala 2.10.3, 2.11.0-M7.