Tender-loving-care: Travis Edition (#930)

Quality of life updates for travis here:
* Use a pub cache to make "pub get" a bit faster.
* Only run travis on [master], to avoid 2 builds per PR
diff --git a/.travis.yml b/.travis.yml
index 0943742..d38d639 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -7,3 +7,11 @@
 env:
   - LINTER_BOT=main
   - LINTER_BOT=benchmark
+
+# Only building master means that we don't run two builds for each pull request.
+branches:
+  only: [master]
+
+cache:
+  directories:
+    - $HOME/.pub-cache