external-config plugin is compiled with Java8

[sbglasius]

external-config plugin is compiled with Java8

[andersaaberg]

I realised that Bintray is only for publishing the plugin to https://plugins.grails.org and not to update the actual plugin code, so I made a normal pull request instead: #18

[andersaaberg]

I can see that Travis Continious Integration intends to use Java7 for releases ("jdk_switcher" @ https://travis-ci.org/sbglasius/external-config/builds/211794021), but it still fails to produce Java7 artifacts so that is the root cause of this issue. I hope that someone can fix that by debugging the Travis process - I cannot reproduce the problem locally.

Setting sourceCompatibility is still a good idea, because it will prevent people from committing Java8 specific code and first realizing the mistake when they have pushed it and Travis Continious Integration fails.

[andersaaberg]

We also need to update GroovyConfigWriter which this plugin depends on: virtualdogbert/GroovyConfigWriter#3