Add Elixir 1.4 RC1 to travis matrix

Looks like we're getting close to the release of 1.4, so I thought
adding the release candidate to the build matrix now would help with
quick compatability and also hopefully help find any bugs in the
release candidate.

.travis.yml

@@ -4,6 +4,7 @@ elixir:
   - 1.1.1
   - 1.2.6
   - 1.3.1
+  - 1.4.0-rc.1
 otp_release:
   - 18.0
   - 18.1