Facilitate automating release. Enable Travis CI.

This factors out the version of scala-parser-combinators to build against
as `scalaParserCombinatorsVersion`, so that builds/releases can be automated
as follows:

```
     SCALA_VER="2.11.0-M7"
   PARSERS_VER="1.0.0-RC5"
SCALACHECK_VER="1.11.0"

sbt 'set version := "'$SCALACHECK_VER'"' \
    'set scalaVersion := "'$SCALA_VER'"' \
    'set every scalaBinaryVersion := "'$SCALA_VER'"' \
    'set VersionKeys.scalaParserCombinatorsVersion := "'$PARSERS_VER'"' \
    clean test publish-signed
```

It also enables Travis CI -- currently only testing against M6 on Java 6&7.
I'd like to propose building against Scala 2.11.0-SNAPSHOT soon.
(We're implementing/trial running this first at scala/scala-xml and the other modules)