update travis config

- add additional build using elixir 1.5.2 and otp 20.1
- bump 1.4.2 to 1.4.5

.travis.yml

@@ -1,15 +1,25 @@
 language: elixir
 elixir:
   - 1.3.4
-  - 1.4.2
+  - 1.4.5
+  - 1.5.2
 otp_release:
+  - 20.1
   - 19.3
   - 18.3
 matrix:
   # newest/oldest pair
   exclude:
-    - elixir: 1.4.2
+    - elixir: 1.5.2
+      otp_release: 19.3
+    - elixir: 1.5.2
       otp_release: 18.3
+    - elixir: 1.4.5
+      otp_release: 20.1
+    - elixir: 1.4.5
+      otp_release: 18.3
+    - elixir: 1.3.4
+      otp_release: 20.1
     - elixir: 1.3.4
       otp_release: 19.3
 sudo: false