Add maven-compiler-plugin section to pom.xml to fix "... are not supported in -source 1.3 (use -source 5 or higher to enable ..)" messages.

Add maven-compiler-plugin section to pom.xml to fix:
  "... are not supported in -source 1.3 (use -source 5 or higher to enable ..)"
messages as described on:
  http://maven.apache.org/plugins/maven-compiler-plugin/examples/set-compiler-source-and-target.html

This allows to compile neo4j-import on Ubuntu 12.04 LTS 64-bit.

Signed-off-by: Gert Hulselmans <hulselmansgert@gmail.com>