Enforce java version 1.7

When building elasticsearch, we now require to use java 1.7.

Maven will check that before compiling any class. If Java version is incorrect, you will get the following message:

```
[WARNING] Rule 0: org.apache.maven.plugins.enforcer.RequireJavaVersion failed with message:
Detected JDK Version: 1.6.0-65 is not in the allowed range [1.7,).
```

Closes #5428.

pom.xml

@@ -335,6 +335,26 @@
         </testResources>
 
         <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-enforcer-plugin</artifactId>
+                <version>1.3.1</version>
+                <executions>
+                    <execution>
+                        <id>enforce-versions</id>
+                        <goals>
+                            <goal>enforce</goal>
+                        </goals>
+                        <configuration>
+                            <rules>
+                                <requireJavaVersion>
+                                    <version>[1.7,)</version>
+                                </requireJavaVersion>
+                            </rules>
+                        </configuration>
+                    </execution>
+                </executions>
+            </plugin>
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-compiler-plugin</artifactId>