Force the gson library to the last valid version that supports Java 1.6

This fixes
e-mission/e-mission-docs#475 (comment)

@st-patrick, in case you want to review this

plugin.xml

@@ -112,7 +112,7 @@
 		</service>
     </config-file>
 
-    <framework src="com.google.code.gson:gson:+"/>
+    <framework src="com.google.code.gson:gson:2.8.5"/>
     <framework src="com.google.android.gms:play-services-location:$LOCATION_VERSION"/>
     <preference name="LOCATION_VERSION" default="11.0.1"/>