Add travis support for CI (not CD) (#3)

* Adding travis support (hopefully without osx).

Based off of processing 5753, demonstrates the use of Travis to build and test processing Requires use of OpenJDK (processing 5753). Will address processing 2747. Will redirect this to the original processing repo after processing 5753 is resolved.

Note that this supersedes sampottinger/processing's #7 and #70 - this does not include the deploy step.

* Attempt explicit lib addition for linux-based travis build.

* Ask travis for ant optional install.

.travis.yml

@@ -0,0 +1,18 @@
+git:
+  depth: 1
+
+language: java
+
+jdk:
+  - openjdk11
+
+before_install:
+  - sudo apt-get -qq update
+  - sudo apt-get install ant-optional
+
+before_script:
+  - cd build
+
+script:
+  - ant clean
+  - ant build