Added Travis CI settings and fix some code.

* Specified sbt version to 0.13.2(current latest version).
* Added Travis CI settings and build status icon.
* Fix unit test compile error.  
(Refer this build result. https://travis-ci.org/zaneli/scala-xmlsoap-ersatz/jobs/24711756#L253 )
* Modify catching Exception type to IllegalArgumentException.  
(Refer this build result. https://travis-ci.org/zaneli/scala-xmlsoap-ersatz/jobs/24725735#L285 )

I have confirmed build success at my repository.
https://travis-ci.org/zaneli/scala-xmlsoap-ersatz/builds/24726782