Adapt sbt-extras for older versions of sbt
Adapt sbt-extras for older versions of sbt

'addSbt warn' and 'addSbt info' are not defined in older
versions of sbt.

See https://github.com/paulp/sbt-extras/issues/99

kevmoo/travis-build,JuliaCI/travis-build,akoeplinger/travis-build,kevmoo/travis-build,jhass/travis-build,akoeplinger/travis-build,jhass/travis-build,craigcitro/travis-build,JuliaCI/travis-build,craigcitro/travis-build,andyli/travis-build,andyli/travis-build