Add elixir 1.5.1 to travis matrix

In addition to adding elixir 1.5.1 to the matrix for testing also test
with elixir 1.5.1 and otp_release 20.0.

Not sure how to make otp 20.0 work with elixir 1.2.6, 1.3.4, and
1.4.0. I believe most people on 1.5.x will use otp 20 or above.

Author: mikestok

.travis.yml

@@ -6,6 +6,11 @@ elixir:
  - 1.2.6
  - 1.3.4
  - 1.4.0
+ - 1.5.1
+matrix:
+  include:
+    - elixir: 1.5.1
+      otp_release: 20.0
 after_script:
   - MIX_ENV=dev mix deps.get
   - MIX_ENV=dev mix inch.report