fix(travis): add lib cups for updated frappe requirements
diff --git a/.travis.yml b/.travis.yml
index 40afeee..365eb67 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -63,6 +63,7 @@
   - tar -xf /tmp/wkhtmltox.tar.xz -C /tmp
   - sudo mv /tmp/wkhtmltox/bin/wkhtmltopdf /usr/local/bin/wkhtmltopdf
   - sudo chmod o+x /usr/local/bin/wkhtmltopdf
+  - sudo apt-get install libcups2-dev
 
   - cd ~/frappe-bench