Add libgles2-mesa-dev to the list of servo dependencies

This will allow linux running over EGL instead of GLX (see servo/servo#7484), and is currently blocking servo/servo#9044, even though it can be worked out.
servo · bors-servo · Dec 22, 2015 · Dec 22, 2015 · Dec 22, 2015 · 272445d43b92df1043d61bb9d12a46f073bc65b9
commit 272445d43b92df1043d61bb9d12a46f073bc65b9
diff --git a/servo-dependencies.sls b/servo-dependencies.sls
@@ -43,6 +43,7 @@ servo-dependencies:
       {% else %}
       - libglib2.0-dev
       - libgl1-mesa-dri
+      - libgles2-mesa-dev
       - freeglut3-dev
       - libfreetype6-dev
       - xorg-dev