travis: enforce elixir 1.6.6 as build dependency

It seems `rabbitmqctl` requires versions >= 1.6.6.

Signed-off-by: Matteo Cafasso <[email protected]>

Author: noxdafox

.travis.yml

@@ -1,5 +1,5 @@
 language: elixir
-elixir: '1.6.0'
+elixir: '1.6.6'
 otp_release:
   - '20.0'
 script: make tests