Check for Java JRE not just a JDK on OS X.

Change-Id: I2cb382bb54a2a9f7fba235e643a100a0408233f5
Fixes-Issue: CLIENT-505
Reviewed-on: http://gerrit.dmdirc.com/3396
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Chris Smith <chris@dmdirc.com>