Enable running tests in Travis CI

This change removes the custom `script` command, falling back to Travis
CI's default Maven script command, which, per [1] is:

    mvn test -B

The effect is that tests should now be run, which is particularly useful
for incoming pull requests.

See bisq-network#1309 (comment)
for context.

[1]: https://docs.travis-ci.com/user/languages/java/#Maven-Default-Script-Command

.travis.yml

@@ -1,8 +1,6 @@
 language: java
 jdk: oraclejdk8
 
-script: mvn clean install -DskipTests
-
 notifications:
   slack:
     rooms: