Ensure flutter format and fix offending files (#80)

The current Travis script will only print offending files, but won’t fail the build. Added `--set-exit-if-changed` to fix that.

Also, simplified the call from using `grep` and `xargs` to simple `flutter format .`, which recurses directories.

Lastly, fixed offending files that needed `dartfmt`. (But not in the directories that aren’t covered by `travis_script.sh`.

veggieseasons/lib/widgets/veggie_card.dart

@@ -130,9 +130,8 @@ class VeggieCard extends StatelessWidget {
               decoration: BoxDecoration(
                 image: DecorationImage(
                   fit: BoxFit.cover,
-                  colorFilter: isInSeason
-                      ? null
-                      : Styles.desaturatedColorFilter,
+                  colorFilter:
+                      isInSeason ? null : Styles.desaturatedColorFilter,
                   image: AssetImage(
                     veggie.imageAssetPath,
                   ),