patch pom.xml to avoid platform specitic encodings used for plugin metadata

Hi - I patched the pom for the following warning when building with the Atlassian plugin SDK on osx for deployment on a Centos system:

``% atlas-mvn package
..
[INFO] [resources:resources]
[WARNING] Using platform encoding (MacRoman actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] Copying 17 resources``

thanks for keeping this repository up !  Saved me some headaches when upgrading from Jira 4.3 to 5.2