Cross build for Scala 2.13.0

[mkurz]

Just a reminder, should be done as soon as Play 2.7 is final, see playframework/playframework#8998

play2-memcached/build.sbt

Line 8 in 66d7412

crossScalaVersions := Seq("2.11.12", "2.12.7"),

play2-memcached/.travis.yml

Line 5 in 66d7412

# - 2.13.0-M4

[mkurz]

See #72