Use _2.11 as the snapshot binary version, rather then _2.11.0.

So as to avoid:

   unresolved dependency: org.scalacheck#scalacheck_2.11.0;1.11.3: not found

In our 2.12.0-SNAPSHOT PR validation.
Merge pull request #20 from retronym/topic/version-bump

Use _2.11 as the snapshot binary version, rather then _2.11.0.