Suggest sortablegridfield in composer, loosen Travis constraint for it

.travis.yml

@@ -21,7 +21,7 @@ matrix:
 before_script:
   - composer self-update || true
   - git clone git://github.com/silverstripe/silverstripe-travis-support.git ~/travis-support
-  - php ~/travis-support/travis_setup.php --source `pwd` --target ~/build/ss --require undefinedoffset/sortablegridfield:0.6.9
+  - php ~/travis-support/travis_setup.php --source `pwd` --target ~/build/ss --require undefinedoffset/sortablegridfield:~0.6.9
   - cd ~/build/ss
   - composer install
 

composer.json

@@ -14,6 +14,9 @@
         "silverstripe-australia/gridfieldextensions": "^1.1.0",
         "silverstripe/taxonomy": "1.2.x-dev"
     },
+    "suggest": {
+        "undefinedoffset/sortablegridfield": "Allow documents to be reordered via drag-and-drop"
+    },
     "extra": {
         "branch-alias": {
             "dev-master": "2.0.x-dev"