Run Java compatibility tests on Travis.

The test is testing the binary/source compatibility between 3.0.0-beta-4
and the current version (right now it's 3.0.0).

Change-Id: Ic410b535758b38cee7baa99a15a0433734df44e6

.travis.yml

@@ -57,6 +57,10 @@ matrix:
     # tests on jenkins running in parallel.
     - os: linux
       env: CONFIG=cpp_distcheck
+    # The Java compatibility test currently only runs on Linux because it will
+    # fetch pre-built Linux protoc binaries in the test.
+    - os: linux
+      env: CONFIG=java_compatibility
   allow_failures:
     # These currently do not work on OS X but are being worked on by @haberman.
     - os: osx

tests.sh

@@ -195,6 +195,14 @@ build_java_oracle7() {
   use_java oracle7
   build_java oracle7
 }
+build_java_compatibility() {
+  use_java jdk7
+  internal_build_cpp
+  # Use the unit-tests extraced from 2.5.0 to test the compatibilty between
+  # 3.0.0-beta-4 and the current version.
+  cd java/compatibility_tests/v2.5.0
+  ./test.sh 3.0.0-beta-4
+}
 
 build_javanano_jdk7() {
   use_java jdk7
@@ -340,6 +348,7 @@ Usage: $0 { cpp |
             csharp |
             java_jdk7 |
             java_oracle7 |
+            java_compatibility |
             javanano_jdk7 |
             javanano_oracle7 |
             objectivec_ios |