Update fedora install instructions

ZJvandeWeg · ZJvandeWeg · commit f9458598beff · 2015-11-24T09:39:55.000+01:00
As `yum` is now replaced with `dnf`, the install instructions where not up to date.
diff --git a/install.markdown b/install.markdown
@@ -35,6 +35,8 @@ Choose your operating system and tool.
     * Run: `emerge --ask dev-lang/elixir`
   * Fedora 17 and newer
     * Run: `yum install elixir`
+  * Fedora 22 and newer
+    * Run: `dnf install elixir`
   * FreeBSD
     * From ports: `cd /usr/ports/lang/elixir && make install clean`
     * From pkg: `pkg install elixir`
