cross-build against Scala 2.11

This required to disable the specialization of the auto-generated Tuple2
instances, because scalac 2.11.0 produces invalid class files with duplicate
methods:

  java.lang.ClassFormatError: Duplicate method name&signature in class file
  spire/std/OrderProductInstances$$anon$228
add 2.11.0 to the Travis configuration
Merge pull request #246 from larsrh/topic/2.11

cross-build against Scala 2.11