Remove Travis CI configuration

This commit removes the Travis CI configuration as we use Concourse for
this now.

.travis.yml

@@ -1,10 +0,0 @@
-language: java
-jdk:
-  - oraclejdk8
-sudo: required
-cache:
-  directories:
-    - $HOME/.m2
-install: true
-before_script: travis_wait 45 ./mvnw install -q -U -DskipTests=true -Dmaven.test.redirectTestOutputToFile=true
-script: ./mvnw install -q -nsu -Dmaven.test.redirectTestOutputToFile=true