Place a library dependency upon slf4j-api, not slf4j-log4j12.

Quoting http://www.slf4j.org/codes.html#multiple_bindings

  Embedded components such as libraries or frameworks should not
  declare a dependency on any SLF4J binding but only depend on
  slf4j-api. When a library declares a compile-time dependency on a
  SLF4J binding, it imposes that binding on the end-user, thus
  negating SLF4J's purpose. When you come across an embedded component
  declaring a compile-time dependency on any SLF4J binding, please
  take the time to contact the authors of said component/library and
  kindly ask them to mend their ways.

Use slf4j-api as a library dependency and use slf4j-log4j12 for unit
testing.

Closes https://github.com/blair/scala-migrations/issues/1 .

Change-Id: Ia6804ac6b9c55965291ad6a79683929d86577da3
README and CHANGES: update for the 1.1.1 release.

Change-Id: I86c979af96d26a5de747b655f669d62aa029497e
Bump version to 1.1.1.

Change-Id: I69804fdcc09c37ddef21bd6eb540f639c20a878c
Bump version to 1.1.2-SNAPSHOT.

Change-Id: If8e1357ac7d2eb97359a620d2d267360e0c2a0d8