Migrate guava logic to java 8

[HugoMario]

There is an error with dependency classes from guava. But instead fixing it with maven dependency declaration, let's take this chance to migrate that logic to Java 8.

[jmini]

At the moment there is a compile dependency on Guava:

• In AbstractJavaCodegen#sanitizePackageName(String) (line 1098) you are using guava perform this check: Strings.isNullOrEmpty(packageName)
• In DefaultCodegenConfig.removeNonNameElementToCamelCase(String, String) (line 2883) both com.google.common.base.Function and com.google.common.collect.Lists are used.

@HugoMario: I guess you mention rewritting the code for the second use case.

---

In the mean time I had a look a the dependencies tree, guava 15.0 is already a test scope dependency, so I added the same version a compile dependencies.

<dependency>
	<groupId>com.google.guava</groupId>
	<artifactId>guava</artifactId>
	<version>15.0</version>
</dependency>

[jmini]

I think the guava version included in the last released version of this project (1.0.0-rc0) was guava 23.0.

The guava dependency is no longer present in the last SNAPSHOT version used in the current dependencies.

See also my analysis (with screenshot) in #4.

[jmini]

I have missed usage of com.google.common.collect.LinkedListMultimap in JavaClientCodegen. Not sure what you want to do about it.

[HugoMario]

Well, the idea is remove any guava dependency

[jmini]

To replace LinkedListMultimap with java8 code, I found this blog post: http://fahdshariff.blogspot.fr/2017/07/java-8-multimaps.html

Second pull request: #7.

[HugoMario]

Travis has a success build now, but it seems there is a config error.
https://api.travis-ci.org/v3/job/338265006/log.txt

I'll be finding out how to solve that.