Move CI to Travis

I ran into this while adding support for Tomcat 9.

What the CI should be testing, give the number of JDKs and different container versions that we support, we should build with respective jdks and versions:

jdk6 + mvn verify -PTC6
jdk7 + mvn verify -PTC7
jdk7 + mvn verify -PAS7
jdk8 + mvn verify -PTC8
jdk7/8 + mvn verify

The situation has changed since original proposal fixing CI with proper dependencies for tomcat versions. Taking into account support for legacy containers has been dropped in master, so now we have desired combinations:

• jdk7 + jdk8 and default profile (excludes Tomcat 9) -> 3 combos on travis
• jdk 8 + Tomcat 9 profile -> 1 combo on travis

See http://tomcat.apache.org/whichversion.html