Added instruction site for zlib

Author: tarebyte

dev_ruby_setup.haml

@@ -249,7 +249,8 @@
               
               - **Zlib isn't installed?**
                   - We noticed that some people using Ubuntu had issues installing Sinatra because
-                    of a missing dependency. Make sure you install your zlib-headers.
+                    of a missing dependency. Follow [these instructions](https://rvm.io/packages/zlib/)
+                    in order to get your gem installs to work.
 
               - **I was installing XYZ, it says it failed. What should I do?**
                   - Try thinking of your problem in its general terms, then typing them into Google.

dev_ruby_setup.html

@@ -251,7 +251,9 @@ <h2>FAQ</h2>
 
             <ul>
             <li>We noticed that some people using Ubuntu had issues installing Sinatra because
-            of a missing dependency. Make sure you install your zlib-headers.</li>
+            of a missing dependency.
+            Follow <a href="https://rvm.io/packages/zlib">these
+            instructions</a> in order to get your gems installs to work.</li>
             </ul>
             </li>
             <li><p><strong>I was installing XYZ, it says it failed. What should I do?</strong></p>