Fix some Jenkins build issues (elastic/elasticsearch#221)

1) The C++ 3rd party libraries need to be copied to cppdistribution
   before gradle assemble runs
2) Use gradle check instead of gradle test in the build
3) We don't need Maven or pbzip2 any more
4) Make new_version.sh script update the right files
5) Keep a copy of the built plugin after the build

Outstanding TODOs:

1) Versioning needs to be brought in line with Elasticsearch
2) We are building a plugin per platform rather than one containing
   all platforms - we need a step that runs when all platforms have
   completed to create a single plugin
3) Strip the C++ binaries so the uploads aren't so big

Original commit: elastic/x-pack-elasticsearch@0cf32e134f

build.gradle

@@ -35,7 +35,6 @@ subprojects {
     compileTestJava.options.encoding = 'UTF-8'
 
     group = 'com.prelert'
-    version = '2.3.0'
     sourceCompatibility = 1.8
     targetCompatibility = 1.8