travis: No longer run nightly builds (#504)

We never took action on nightly failing, and we prefer to get results
faster and not use up as many of the organisation-wide five build slots.

Closes #498

Author: petertseng

.travis.yml

@@ -13,7 +13,6 @@ sudo: false
 rust:
   - stable
   - beta
-  - nightly
 env:
   - DENYWARNINGS=
   - DENYWARNINGS=1