Add maven enforcer plugin configuration to enforce JDK 8

JDK 8 is now required, but the error on maven build is not so obvious

```bash
$ mvn clean install
[INFO] Scanning for projects...
[INFO] 
[INFO] Using the builder org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder with a thread count of 1
[INFO]                                                                         
[INFO] ------------------------------------------------------------------------
[INFO] Building Spark 2.0.1-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[INFO] 
[INFO] --- maven-clean-plugin:2.5:clean (default-clean) @ spark-core ---
[INFO] 
[INFO] --- maven-enforcer-plugin:1.0:enforce (enforce-maven) @ spark-core ---
[INFO] 
[INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ spark-core ---
[INFO] Using 'UTF-8' encoding to copy filtered resources.
[INFO] skip non existing resourceDirectory /Users/mheuer/working/spark-1/src/main/resources
[INFO] Copying 0 resource
[INFO] 
[INFO] --- maven-compiler-plugin:2.3.2:compile (default-compile) @ spark-core ---
[INFO] Compiling 57 source files to /Users/mheuer/working/spark-1/target/classes
[INFO] -------------------------------------------------------------
[ERROR] COMPILATION ERROR : 
[INFO] -------------------------------------------------------------
[ERROR] Failure executing javac, but could not parse the error:
javac: invalid target release: 1.8
Usage: javac <options> <source files>
use -help for a list of possible options

[INFO] 1 error
[INFO] -------------------------------------------------------------
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 4.005 s
[INFO] Finished at: 2014-05-19T11:19:03-06:00
[INFO] Final Memory: 8M/245M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:2.3.2:compile (default-compile) on project spark-core: Compilation failure
[ERROR] Failure executing javac, but could not parse the error:
[ERROR] javac: invalid target release: 1.8
[ERROR] Usage: javac <options> <source files>
[ERROR] use -help for a list of possible options
[ERROR] -> [Help 1]
[ERROR] 
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR] 
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoFailureException
```

This pull request adds a maven enforcer plugin configuration that fails the build if JDK 8 is not being used and displays a better error message:
```bash
$ mvn clean install
...
[INFO] --- maven-enforcer-plugin:1.3.1:enforce (enforce-java) @ spark-core ---
[WARNING] Rule 0: org.apache.maven.plugins.enforcer.RequireJavaVersion failed with message:
Detected JDK Version: 1.7.0-55 is not in the allowed range [1.8,).
```