Fix Scala and Java versions in Travis config

Problem

We no longer intend to support Java 6, and it should not be included in the
Travis config. Also the use of `+test` in the SBT command causes the tests to
be run against all Scala versions in the SBT config, not Travis's version.

Solution

Remove OpenJDK 6 and the plus sign in the SBT command, and add Scala 2.11.2.

Result

Tests run as intended.