Added Ruby to Travis testing.

- Added RVM-based Ruby test driver that tests MRI and JRuby.
- Fixed JRuby compilation (at least in my current setup): force source
  version to 1.6 (Java 6) to allow generics and annotations.
- Modify the skipped JRuby JSON tests so that the exit code is 0 (skip()
  results in a failing exit code from `rake test`). An upcoming PR
  should fix JSON under JRuby in general soon.
diff --git a/ruby/pom.xml b/ruby/pom.xml
index 01f0e16..247b243 100644
--- a/ruby/pom.xml
+++ b/ruby/pom.xml
@@ -61,6 +61,14 @@
                     </execution>
                 </executions>
             </plugin>
+            <plugin>
+              <groupId>org.apache.maven.plugins</groupId>
+              <artifactId>maven-compiler-plugin</artifactId>
+              <configuration>
+                <source>1.6</source>
+                <target>1.6</target>
+              </configuration>
+            </plugin>
         </plugins>
     </build>
     <dependencies>
@@ -78,7 +86,7 @@
         <dependency>
             <groupId>com.google.protobuf</groupId>
             <artifactId>protobuf-java</artifactId>
-            <version>3.0.0-alpha-3-pre</version>
+            <version>3.0.0-alpha-2</version>
         </dependency>
     </dependencies>
 </project>