Fix .travis.yml

Fixed the yaml file to support `0.6` and to allow for failures in
nightlies.

Author: Arda Aytekin

.travis.yml

@@ -5,7 +5,12 @@ os:
 julia:
   - 0.4
   - 0.5
+  - 0.6
   - nightly
+matrix:
+  fast_finish: true
+  allow_failures:
+    - julia: nightly
 notifications:
   email: false
 #script: # the default script is equivalent to the following