Add configuration for building on Travis CI

This commit adds the necessary configuration to build the project
on Travis CI. First, the main project is built using Java 7 then the
samples are built using Java 8.

Closes gh-58

.travis.yml

@@ -0,0 +1,6 @@
+language: java
+script:
+  - jdk_switcher use oraclejdk7
+  - "./gradlew build"
+  - jdk_switcher use oraclejdk8
+  - "./gradlew buildSamples"