PARQUET-121: Allow Parquet to build with Java 8

There are test failures running with Java 8 due to http://openjdk.java.net/jeps/180 which changed retrieval order for HashMap.

Here's how I tested this:

```bash
use-java8  
mvn clean install -DskipTests -Dmaven.javadoc.skip=true 
mvn test
mvn test -P hadoop-2
```

I also compiled the main code with Java 7 (target=1.6 bytecode), and compiled the tests with Java 8, and ran them with Java 8. The idea here is to simulate users who want to run Parquet with JRE 8.
```bash
use-java7
mvn clean install -DskipTests -Dmaven.javadoc.skip=true 
use-java8 
find . -name test-classes | grep target/test-classes | grep -v 'parquet-scrooge' | xargs rm -rf
mvn test -DtargetJavaVersion=1.8 -Dmaven.main.skip=true -Dscala.maven.test.skip=true
```
A couple of notes about this:
* The targetJavaVersion property is used since other Hadoop projects use the same name.
* I couldn’t get parquet-scrooge to compile with target=1.8, which is why I introduced scala.maven.test.skip (and updated scala-maven-plugin to the latest version which supports the property). Compiling with target=1.8 should be fixed in another JIRA as it looks pretty involved.