Fix twitter link in install flow

Fixes #400

// FREEBIE

options.html

@@ -26,7 +26,7 @@
         <p>First, install <a class='link' href='https://play.google.com/store/apps/details?id=org.thoughtcrime.securesms' target='_blank'>Signal</a> on your Android phone.<br /> We'll link your devices and keep your messages in sync.</p>
         <div> <a class='button step3'>I have Signal for Android</a> </div>
         <p>
-        <a class='link' href='https://twitter.com/whispersystems'>Follow us</a>
+        <a class='link' href='https://twitter.com/whispersystems' target='_blank'>Follow us</a>
         for updates about multi-device support for iOS.
         </p>
         <span class='dot step1'></span>