remove travis button since it links to old domain

this is a workaround since the alabaster theme doesn't support
the new travis domain

this would no longer be necessary with
sphinx-doc/alabaster#147

Author: ajyoon

doc/source/conf.py

@@ -90,7 +90,6 @@
     'fixed_sidebar': True,
     'github_banner': True,
     'github_button': False,
-    'travis_button': True,
     'show_powered_by': True,
 }