Add Indigo test matrix in travis configuration (#50)

Author: jihoonl

.travis.yml

@@ -19,11 +19,14 @@ notifications:
 env:
   matrix:
     - ROS_DISTRO="kinetic"   ROS_REPOSITORY_PATH=http://packages.ros.org/ros/ubuntu  USE_DEB=true
+    - ROS_DISTRO="indigo"    ROS_REPOSITORY_PATH=http://packages.ros.org/ros/ubuntu  USE_DEB=true
     - ROS_DISTRO="kinetic"   PRERELEASE=true PRERELEASE_DOWNSTREAM_DEPTH=0
+    - ROS_DISTRO="indigo"   PRERELEASE=true PRERELEASE_DOWNSTREAM_DEPTH=0
 
 matrix:
   allow_failures:
     - env: ROS_DISTRO="kinetic"   PRERELEASE=true PRERELEASE_DOWNSTREAM_DEPTH=0
+    - env: ROS_DISTRO="indigo"   PRERELEASE=true PRERELEASE_DOWNSTREAM_DEPTH=0
 
 branches:
   only: