Make sure plugins are compiled for Java 1.7.

Change-Id: Ibe571916d24aa437c38b94c037f61fb8d342b444
Reviewed-on: http://gerrit.dmdirc.com/3444
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager

build.xml

     <property name="plugins.build.metadata" location="${plugins.build}/metadata"/>
     <property name="plugins.dist" location="dist"/>
     <property name="plugins.lib" location="lib"/>
+    <property name="plugins.source.level" value="1.7"/>
+    <property name="plugins.target.level" value="1.7"/>
 
     <import file="build-ivy.xml" />
     <import file="build-tests.xml"/>
     <target name="compile" depends="-init-compile">
        <javac srcdir="${plugins.src}" destdir="${plugins.build.classes}"
               classpathref="plugins.classpath" includeantruntime="false"
-              debug="true" deprecation="true">
+              debug="true" deprecation="true"
+              source="${plugins.source.level}"
+              target="${plugins.target.level}">
            <compilerarg value="-Xlint:all"/>
            <compilerarg value="-Xmaxwarns"/>
            <compilerarg value="1000"/>