Make the build aware of Scala 2.11 modularization

Cherry-picking 72c0e19
update 2.11 prerelease version
Avoid inferring by-name type argument to function1Covariant

Cherry-picking 502c9e8
Test compilation against 2.11.0-M6 in Travis
comment explaining the backport of `function1CovariantByName`
make `typelevel` compatible with 2.11
bump scalacheck version

Cherry-picking a5a4efb
Merge pull request #568 from larsrh/topic/build-2.11

Scala 2.11 compatibility (for 7.0.x series)