fixed image link in installation instructions.

docs/INSTALLATION.md

 If you have no JDKs installed at all (e.g. you have a fresh install of Mac OS X 10.10 [Yosemite]),
 the OS presents a dialog:
 
-![To use the "java," command-line tool you need to install a JDK.  Click "More Info..." to visit the Java Developer Kit download website.](img/mac-osx--install-java-dialog.png)
+![To use the "java," command-line tool you need to install a JDK.  Click "More Info..." to visit the Java Developer Kit download website.](http://x.exercism.io/v3/tracks/java/docs/img/mac-osx--install-java-dialog.png)
 
 Clicking on the "More Info..." button takes you to the [Oracle OTN](http://www.oracle.com/technetwork/java/javase/downloads/index.html).